Fill in the blank in the sentence below with the most natural form of the word in brackets. The response is case sensitive.

An seller is waiting for customers at the markets. (onion)

An onion seller is waiting for customers at the market.

Explanation:

So according to the expectation of the question, the answer "onion", "o" is not capitalized.

Reason:

"Onion" is a noun. In simple terms noun represent "name, place, animal things".

So noun should be capitalized but it also depends on the type of noun. All the vegetable names, fruits name, etc comes under common noun.  

Example: cat, women, uncle

So, mostly common nouns are not capitalized.  

But Proper nouns are usually capitalized.

Example: Pacific Ocean, America

dinner and a movie is the wrong answer !