The correct answer is: E. No error
The sentence is grammatically correct. The word “whatever” is used to introduce a clause that expresses a general idea. The word “that” is used to introduce a clause that refers to a specific thing. The word “undertake” means to take on a task or responsibility. The word “put” is used to mean to place something in a particular position. The word “best” means the most excellent or effective. The word “efforts” means attempts or endeavors. The word “in it” means in the thing that is being discussed.
The sentence can be paraphrased as follows: “No matter what task you take on, always give it your best effort.”