Answer: Barbara Bush focused much of her energy on a cause near to her heart: literacy.
Explanation: In the first two options, the colon is being misused because it is fragmenting essential elements of a same clause (direct object "much of her energy" from complement "on a cause..." / prepositional phrase "on a cause" from prepositional phrase "near to her heart" modifying it.) In the third option, the comma should not be used to introduce the one-item list "literacy."
