The correct option is B seeing him
Let’s examine the sentence: “He must be the director. I remember ____ in school.” The sentence specifies ‘memory’ of ‘seeing’ and not the action of ‘seeing,’ and so it requires a noun or a gerund in the blank. The gerund always has the same function as a noun (although it looks like a verb). The gerund ends in ‘ing.’ As Option C fulfils this requirement, so it is the correct answer. All the other options cannot be used as they would make the sentence meaningless.