May I know (1)/ to who (2)/ I am speaking ? (3)/ No error. (4)

Answer

2

Explanation

whom will replace who because - who refers to the subject of a sentence whereas whom refers to the Object of a sentence. If the Pronoun can be replaced by he, we use who and if it is replaced by him, we use whom
Look at the examples given below:
Who is the best footballer in the world ?
`underset("Subject")underset(darr)("Who")`is the best footballer in the world ?
Subject `rarr` can be replaced by he.
Object `rarr` can be replaced by him ?
Hence, to whom is the right usage.