1 out of 1 members found this post helpful.
Correct answer is D. In this sentence "looking" is part of verb phrase "was looking"- past continuous tense (was/were verb+ing). So, you have to have noun ("he") before that phrase.
But for better understanding of grammar, you should divide this sentence in two simple sentence. "In 1987 a Canadian astronomer, Ian Shelton, spotted a supernova" while "he was looking at some photographs of the stars".
And if you want to use "looking" as noun (verb+ing= gerund) you should have verb near it - but we do not have that option.
|Going to parties is fun.
||I enjoy reading.
|Gerund is subject
||Gerund is object
I really hope that my explanation will be helpful!!!