The paragraph refers to an activity that was completed last week. This is past tense. So, past participle of the verb will be used.
"Looking after" is the continuous form of the verb. This is incorrect.
"Looked after" in option B is correct as it indicates the action that the science club completed at the time of the science fair.
"To look after" is a phrase, which has no application reference in its prepositional form.
"Has looked after" is in tense disagreement with the context.
Thus, option B is the correct answer.