Hi Supriya, 

Pourquoi il a soif ?  

is very informal and it isn't something you would write, except if you were writing informal dialogue in a story.

 'Pourquoi est-ce qu'il a soif ?

is still informal and the best-written question would be -

Pourquoi a-t-il soif ?

Here is the relevant Kwiziq lesson -

https://kwiziq.learnfrenchwithalexa.com/revision/grammar/forming-questions-by-inverting-in-the-present-tense-il-elle-on-forms

Bonne Continuation!