Are " How to do?" and "What to do?" grammatically correct? [closed]

Are " How to do?" and "What to do?" grammatically correct?

For example: A - I left your book in the school today. B - How to do ? OR What to do?

Solution 1:

What to do? is the correct option here.

The interrogative pronoun is here used as the direct object of the infinitive to do. A listener makes the necessary insertion: What [is there] to do?

Examining the other option, one sees that how is an interrogative adverb, but as the inquirer is asking what there is to do, not how something is to be done, this option does not fit here.

Note that NEITHER OPTION IS A COMPLETE SENTENCE.