Which of the following is a grammatically CORRECT sentence?

The first part of the sentence deals with the correct usage of the word "had" and "tell".

"had better" is a semi modal verb (used to express need or necessity) used to give strong advice and is the correct usage.
Since "had better" implies advice to be followed, the verb that follows cannot be in the past tense, so the correct selection is "tell".

In the second part of the sentence, the choices are between will and would, considering the first part of the sentence, we need future perfect simple tense, in this case, "will".

Hence the correct option is C.