'This is the richest fraternity on the campus; therefore Mr. X who is a member of this fraternity must be one of the richest young man on the campus.' Which fallacy is involved in this argument?

The argument takes what is true of the whole fraternity, its being the richest, and forces it onto an individual member.

A property of a collective need not belong to each of its parts.

The fraternity can be richest overall while a given member is not among the richest.

Inferring from whole to part in this way is the fallacy of division.

So the reasoning about Mr. X commits the fallacy of division.