Answer :
The answer is a. Conjecture is defined as a guess, opinion, theory, or the formation of such theory or opinion. Discourse is a written or spoken debate. An interjection is an abrupt remark, made especially as an aside or interruption. A precursor is a person or thing that comes before something else (forerunner). Thus, the only word that would make sense in this sentence is conjecture, especially when you take into account that the sentence states that it was later on proven correct. Only a conjecture can be proven correct.
