What word completes the sentence: "Not at all it's _____ a matter of time before we really begin to feel the effects."?

The word "merely" effectively completes the sentence, as it implies that the situation at hand is not significant or substantial. The phrase "Not at all it's merely a matter of time" suggests that the speaker believes the effects will come sooner rather than later, but also downplays the importance of that waiting period. This usage communicates a sense of inevitability without overemphasizing it, reinforcing the notion that time is the only variable at play.

In contrast with the other options: “truly” would suggest a strong affirmation of the statement that follows, "hardly" would imply that the matter is insignificant, and "definitely" would tend to suggest certainty and insistence about the matter, which does not align with the overall tone intended in the original sentence.