Question

Fill in the blank with the most appropriate word.
A: What started the fire?
B: Nobody knows __________.
a) who started the fire
b) what started the fire
c) what starts the fire
d) who’d started the fire

Answer 1

Hint: The sentence suggests that the person talking is specifying which object he is talking about to someone. When specifying an object, a specific ‘wh-’ word is to be used.

Complete answer:
 In English grammar, a ‘wh-word’ is one of the function words used to begin a wh- question: what, who, whom, whose, which, when, where, why, and how. Wh- words can appear in both direct questions and indirect questions, and they are used to begin wh- clauses.
The sentence is structured in such a way that the meaning inferred becomes that the cause of the fire starting was because of an inanimate object, due to the word used, i.e. ‘what’. So, the second sentence should also follow suit.

Now we know what we are looking for. Let’s look at our options one by one-
a) who started the fire - This phrase starts with the word ‘who’. However, the word ‘who’ is used to refer to a person. The whole phrase is wrong for the given sentence. This is not the required answer. So, this is an incorrect option.
b) what started the fire - This phrase starts with the word ‘what’. The word ‘what’ is used to refer to an object. The whole phrase is right for the given sentence. This is the required answer. So, this is the correct option.
c) what starts the fire - This phrase starts with the word ‘what’. The word ‘what’ is used to refer to an object. However, the verb is in the wrong tense. The whole phrase is wrong for the given sentence. This is not the required answer. So, this is an incorrect option.
d) who’s started the fire - This phrase starts with the word ‘who’. However, the word ‘who’ is used to refer to a person. The whole phrase is wrong for the given sentence. This is not the required answer. So, this is an incorrect option.

Note:
Identify the structure of the sentence, and the parts of speech used. They are in a particular order. Now, use the trial and error method and see which option best fits into the sentence.