In the following passage, some words have been deleted. Read the passage carefully and select the most appropriate option to fill in each blank.
[Same Passage as Above]

Select the most appropriate option to fill in blank no. 2.

The noun phrase "a matter of [something]" is a standard idiomatic expression used to describe a situation that involves a particular feeling or state (e.g., "a matter of great hope and relief"). None of the other choices (object, topic, stuff) fit grammatically or idiomatically into this framework.