In the addition of hbr to 1-butyne the electrophile in the first step of the mechanism is

            In the addition of hbr to 1-butyne the electrophile in the first step of the mechanism is Hydrogen atom of HBr.

Explanation:
                   In this reaction first of all HBr approaches the triple bond. A Pi Complex (weak inter-molecular interactions) is formed between the two molecules. And the triple bond attacks the partial positive hydrogen atom creating a negative charge on Bromine along with positive charge on itself (Sigma Complex). In second step the negative Bromide attacks the positive carbon of Butyne.

The total number of electrons is 14

The answer to this is the 3rd choice!