which (used for choices or selections)

कौन सा

English Usage: Which dress do you prefer for the party?

Hindi Usage: आप पार्टी के लिए कौन सा ड्रेस पसंद करते हैं?

which (used to specify a particular thing or person)

which (used in relative clauses)

कौन

English Usage: Which book do you want to read?

Hindi Usage: आप कौन सी किताब पढ़ना चाहेंगे?