I agree, the answer is a. The colon after attitude helps separate the cause and the explanation. All the other answers don't make sense because they didn't separate it properly.
A. There is a simple explanation for my sister's attitude: She is too young to understand.
The correctly punctuated sentence is option A: 'There is a simple explanation for my sister’s attitude: She is too young to understand.' It uses a colon correctly to introduce an explanation of the sister's attitude. The other options contain misplaced punctuation or grammatical errors, making them incorrect.
;