Universal instantiation: Given a universal generalization (an ∀ sentence), the rule allows you to infer any instance of that generalization. It's a valid rule of inference from a truth about each member of a class of individuals to the truth about a particular individual of that class.
A, B and D have the same premise. From ∀x Cube(x), infer Cube(b). But C, even if it's a logical argument, is not demonstrating universal instantiation. It's using a quantifier for the certain amount of people in that class.
