who, what – कः, का, किम्

कः meaning (masculine) who, what
का meaning (feminine) who, what
किम् meaning (neuter) who, what

Notes:
All these three means the same in sanskrit. Only gender varies. According to the context it means who or what.
Even for non-living things कः, का are used. Same way किम् is used for living beings too.
These can be used only in singular (एकवचनम्)

Usage:
सः कः? – Who is he?
रामः कः? – Who is Ram?
नृपः कः?- Who is the king?
ॠषिः कः – Who is the sage?
कः आगच्छति? – Who is coming?
पुस्तकम् कः प्रेषयति? – Who is sending the book?
भारत देशस्य कः प्रधानमन्त्री? – Who is the Prime Minister of India?
भवान् कः? (पु.)-Who are you?
वाटिकायाम् कः क्रिडति?-Who is playing in the park?
गानम् / गीतम् कः गायति? – Who is singing the song?
बालकः कः? – Who is the boy?
त्वम् कः? – Who are you?
समयः कः? – What time is it?
नियमः कः? – What rule?
कः देवालयः? – What temple?
कः कार्यालयः? – What office?
भवती का? (स्त्रि.)-Who are you?
त्वम् का? (स्त्रि)-Who are you?
सा का? – Who is she?
बालकः किम् खादति? What is the child eating?
युवकः किम् नयति? – What is the youth taking?
किम् मित्रम्? – Who is friend?