Explanation:

Correct Answer is (d).Two very important keywords are here in the sentence - ‘Trusted’ and ‘Knowledgeable’. What do we generally do when some trusted and knowledgeable gives a piece of advice to us? We ‘ponder’ over it. 
Ponder (Verb):
To think about (something) carefully, especially before making a decision or reaching a conclusion.
Clearly, ‘pondered’ is the best suited word here for the blank.
‘Think’ can’t be chosen here because it’s in present form whereas the word required for the blank has to be in past form.
Option D is hence the correct answer.