A recipe requires 2 1/2 (mixed number) cups of flour 2 3/4..

Let's start by evaluating how many cakes we can make considering one ingredient at a time.

Flour: If each cake uses 2.5 cups, we can make 2 cakes with 5 cups, and 6 cakes with 15 cups. Right away, we can eliminate everything except A and B.

Sugar: If each cake uses 2.75 cups, we can make 2 cakes with 5.5 cups, and 6 cakes with 16.5 cups. However, we only have 16 cups, so we can only make 5 cakes. We can eliminate B, leaving us with A, which must be our answer.

We can confirm by repeating the process with milk.

Milk: If each cake uses 1.333333 cups, we can make 3 cakes with 4 cups, and 6 cakes with 8 cups. So sugar is still our limiting factor.