Pita is respectful form of word for father. Baap is not so respectful form for father. Since Pita is used, the respectful form of "uska" has to be used. That's why - उसके पिता If baap is used, the right usage is उसका बाप.

As per Indian tradition and culture hindu treat their parents as God(Father) & Godess(mother) as due to their efforts only we came in this life. So we always add respectful terms when we are talking about our mother and father.

A word of caution. बाप is, in fact, less respectful. People might take offense (or consider you as someone with low moral value) if you refer their/your father as बाप. पिता is kind of purest usage derived from Sankrit & not used as often in colloquial conversations as much as पिताजी or पापा are used.

Im not sure about this But as far as i know "Ka" and "ki"to shown whos "gender belonging" ka identified for maskulin Ki identified for feminim And ke , not two of both. Uski pita = her father (feminim) uska pita = his father (maskulin)