At the very outset let me make it amply clear that the styles of expression among various languages differ considerably. Native speakers will express their ideas( sometimes with a lot of regional slangs) in such a way that is easily and clearly understood among themselves.  Their expressions cannot be translated word for word to get the expected idea/ meaning by the people of other nationalities.  I have come across a lot of people asking for clarifications in the above case.  I would venture to outline it which, I believe, should clarify your doubts to some extent.

Example:-  ” I am the 4th son of my parents.”  (How will you frame a question to get an answer for the above?) Now you can ask like:

• How many sons do your parents have? , which one are you?
• Answer : I am the 4th son of my parents. ( according to the different situations, there can be variations in the answers depending on what is asked for).
• Which one are you?
• I am the engineer, now working in Cognizant.
• I am the doctor, at Mission Hospital.
• I am the shop-keeper,  I am the sportsman etc etc. ( Thus the word ‘WHICH’ will decide what is asked for).
• Thus :- ” If E.M.S.Namboodiripad  was the first Chief Minister of Kerala, Pinarayi Vijayan is which one?
• OR (in a roundabout way) ” Where does Pinarayi Vijayan fall in the sequence of Kerala Chief Ministers?”
• OR ” Which one is Pinarayi Vijayan among Kerala Chief Ministers?”