The correct option is C eavesdropping
To eavesdrop means to secretly listen to somebody’s conversation, hence option C is the correct answer because it is the most suitable substitute for the given statement.
Option A is incorrect because “overhearing” means hear (someone or something) without meaning to or without the knowledge of the speaker, which is not done secretly, hence incorrect.
Option B is incorrect because “to pigeonhole “ means to assign to a particular category, typically an overly restrictive one, which means to categorise, hence it is incorrect.
Option D is incorrect because “spying “ means work for a government or other organization by secretly obtaining information about enemies or competitors, hence it is incorrect as spying would include eavesdropping in it.