Why China has less population than India?
Shouldn't it be like:
Why has China less population than India?
OR
Why do China has less population than India?
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Sign up to join this communityWhy China has less population than India?
Shouldn't it be like:
Why has China less population than India?
OR
Why do China has less population than India?
English is dependent on word order to get meaning across.
We simply do not form questions that way (note, the clause structure would be fine as a declarative, such as the title of a book or article).
Instead we would structure the question this way, most commonly in American English:
Why does China have less population than India?
The following is also grammatical, but I'd never say it like this:
Why has China less population than India?
The second one sounds British or old fashioned.
I was throwing a brick.
Was I throwing a brick?
I am awesome.
Am I awesome? (verb is to be so do not needed)
She ran away quickly.
Did she run away quickly?
He had ran away very quickly.
Had he ran away very quickly? (have is an auxillary in to have run so do not needed)
I have 3 marbles.
Do you have 3 marbles? (have is NOT an auxillary so do is needed)
Have you 3 marbles? (Agree with @NES, sounds very formal, British or old fashioned)
I went to the park.
When did I go to the park? (second words in auxillary verbs do not change with the first word)
China has less population than India.
Why does China have less population than India.
They had been walking for 2 hours.
Had they been walking for 2 hours? (have is an auxillary in have been walking so do not needed)
None of your alternatives is correct. For a start: population is not more/less, it is higher/lower.
Secondly, none of the three sentences uses a valid interrogative form. The only valid one is does + have in this case (because, as someone already pointed out, in this case "have" is not auxillary), not do + has. The third person goes in the auxillarry verb, i.e:
Why does China have a lower population than India?