I have the following phrase:
Which of the activities in the photos look the most fun?
Can you explain why it's "look" and not "looks"?
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Assuming that you found the question here, we can reasonably guess that the authors want readers to be free to choose more than one activity as being (among) the most fun.
In that case, which is a relative pronoun representing a plural subject (activities). We can see why look belongs in its plural form if we replace the pronoun which with [which + the name of the subject] and arrange the sentence into an imperative instead of its question form:
Tell your partner which activities, of the activities in the photos, look the most fun.
A possible response, using a plural subject with look as a present simple verb in plural form might be:
Gardening and eating look the most fun to me. I love growing and eating food.
Subjects and verbs generally need to agree in number to be accepted by some readers and listeners.
For example, I lives in a zoo is something that many would accept only from an animal.
In certain cases, it is relatively more common for native speakers to say things where subjects and verbs disagree in number, and oftentimes, especially in speech, listeners tend to accept them with no problem. This is especially true when a noun from the differing number case appears between the subject and the verb:
[Section under construction!]
If the intention is to ask the reader to choose only one activity as the single most fun, then the question is really which one, and in that case, look should agree with this subject and be made singular in form by adding s.
Which [one] of the activities in the photos looks the most fun?
But, as we've observed, which can be a pronoun representing a subset of two or more of the activities. In that case, the word most indicates that a selected subset would be compared to one or more other subsets.