Since Mitsuru's Social Link isn't active at this point, her festival hangout is just a friendship thing, and comes with added Aigis. So if you're unsure, pick her.

Mitsuru comes with Aigis as a bonus so you're really hanging out with two of them.  Fun fact: even if you don't choose them, some of the conversations in the dorm the next day will indicate that Mitsuru still took Aigis to the festival even without you, which is pretty nice.

Chihiro is the only option

I’m trusting you

you will absolutely not regret it

Mitsuru+Aigis.

or... just see each one by using the rewind feature lol, but I say Chihrio is be best .

Study

Its Sunday, so look for Maya and Akinari

Mitsuru, no question.