せんぱいが好きな食べ物は、なんですか？
I come across this sentence in a web novel, I think it is supposed to be せんぱいの好きな食べ物は、なんですか？Why is せんぱい marked with が here? Is there a different use of the word すき other than the comman usage: "先輩は食べ物が好きだ" that mark the person other than the object being described as favourite? Is it an example of the usage "in love (with)" in the EN-JP dictionary?
With adjectives like 好き it is possible to say the following
[sentence 1] りんごがすきです
[sentence 2] わたしがすきです
Sentence 1 should be relatively easy to understand
Someone likes apples.
Sentence 2 isn't particularly clear when completely divorced from context.
It could mean,
Someone likes me
I would say that without any further context, this is the default reading.
But it could also mean,
I like something.
There's a lot of discussion on this site about the difference between は and が in this sort of context. I'll leave that discussion aside except to say that as a stand alone sentence
わたしがすきです
Either sounds like I am liked by someone or I am the sole individual within some group who likes something.
But, we don't have a stand-alone sentence here.
We have instead
せんぱいが好きな食べ物は、なんですか
せんぱいが好きな is a relative clause modifying 食べ物. Since foods typically don't have a preference for who's going to eat them (unless your in some strange alternate universe of anime, sci-fi, etc), the most natural way to construe this is that 食べ物 is what is being liked.
That means that the relative clause
せんぱいが好きな
functions much like sentence 2 above in the sense where it is せんぱい is the one who is liking something.
But, there is one important caveat here. The は-が distinction I mentioned above is null here. That's because は as the topic marker cannot be embedded in a relative clause (when は is found in a subordinate clause, then は is not marking the topic but usually functioning contrastively).
So this が in the relative clause marks the subject who likes some kind of food. Unlike sentence 2 above, in a relative clause, the reading is neutral: that is, nothing is to be construed as せんぱい being the only individual who likes something.
You could indeed say
せんぱいの好きな食べ物は、なんですか
But if you're thinking this の is a possessive marker indicating
せんぱいの食べ物
then you are unfortunately mistaken. A native speaker is not naturally inclined to hear it like this. That is because の can be used in relative clauses (under special conditions which apply here) to stand in for the subject marker が.
For details on subtopics I've brought up:
So you say you know about relative clauses. Presumably then you would be happy with a sentence like
せんぱいが食べた食べ物は何ですか。
What food did senpai eat?
In this sentence せんぱいが食べた modifies 食べ物 to describe the food. It's the food that 'senpai ate'.
せんぱいが食べ物を食べた --> せんぱいが食べた食べ物
Senpai ate food --> the food senpai ate.
The object gets promoted to the head noun of the relative clause.
The same thing happens with your sentence: せんぱいが好きな食べ物. This is "the food that senpai likes".
In this case we have
せんぱいは食べ物が好きだ --> せんぱいが好きな食べ物
Senpai likes food --> the food senpai likes
In this case it is the subject rather than the object that has been promoted to the head noun, but that's allowed too.
There are a couple of minor oddities to watch out for with this one. せんぱいは has become せんぱいが because は is not (usually) allowed in relative clauses. And, だ becomes な because you changed the predicative form of the na-adjective into the attributative (is that a real word?) form.
Finally, you say you would be happier if the sentence was せんぱいの好きな食べ物は、なんですか. This would also be perfectly grammatical, but I suspect not for the reason you think. の can replace が in relative clauses. See this link for example.
I guess you wanted to treat 好きな食べ物 as a noun and then modify it with せんぱいの to get "senpai's liked food". I'm not sure if that is a valid way to look at the grammar or not. Perhaps someone else can weigh in on that.
