To answer this question, you need to understand idioms and their meanings.
Let's go through each option to understand why it is correct or incorrect:
Option A) "as awkward as a cow on roller skates" - This option is incorrect because it means someone who is very clumsy or awkward, which does not fit the context of the sentence.
Option B) "a kangaroo court" - This option is incorrect because it refers to a mock court where the principles of law and justice are disregarded. It does not convey the meaning of being easily frightened.
Option C) "as quiet as a mouse" - This option is incorrect because it means someone who is very quiet or silent. It does not convey the meaning of being easily frightened.
Option D) "a scaredy-cat" - This option is correct because it refers to someone who is easily frightened or timid. It fits the context of the sentence, where the little boys think their friend is easily frightened.
The correct answer is D.