Solution:(a) The boy told his mother that he saw the tallest giraffe at the zoo.(Correct)(b) The boy told his mother that he saw the most taller giraffe at the zoo. (Incorrect Degree)
(c) The boy told his mother that he saw the most tallest giraffe at the zoo. (Incorrect Degree)
(d) The boy told his mother that he saw the more taller giraffe at the zoo. (Incorrect Degree)