The correct answer is A. There is no error in colon usage. Colon is used when enumerating a list of items which also requires eliminating the verb, like the example below.
WITH COLON: Sky and Saab like to buy the following for their science project: a piece of plywood, electric wires, and a bulb.
WITHOUT COLON: The materials Sky and Saab like to buy for their science project are a piece of plywood, electric wires, and a bulb.