Correct option is B
The sentence "You will get wet if you will go out in the rain." contains a conditional sentence error. In English, the correct structure for a conditional sentence (Type 1 conditional) involving a future possibility does not use "will" in the 'if' clause. The 'if' clause should be in the simple present tense when referring to a future condition.
The corrected sentence should read: "You will get wet if you go out in the rain."
So, the error is in the use of "will" in the 'if' clause, which should be removed to correct the sentence. Therefore, the answer is: (B)