The noun 'sunset' is an atmospheric phenomenon, it's unique and hence an article is not used before it. 'The' modifies the noun by stating 'this particular sunset', which isn't correct because the noun is unique, there is only one phenomenon called sunset. Hence 'the' needs to be dropped out, option A is incorrect. The sentence with the omission of the article is in option C, hence C is correct. In options B and D, the indefinite articles 'a' and 'an' are used respectively. Indefinite articles modify nouns that are not specific, one of many such nouns. Since 'sunset' is a unique phenomenon, there isn't one of many sunsets, these articles are incorrect here, and hence options B and D are incorrect.