Topic: Antonyms, Subject: English Grammar (ইংরেজি গ্রামার)

Best Answer Explained in Below-:

The term "deleterious" refers to something that is harmful or damaging, often in a subtle or unexpected way. Therefore, its antonyms would naturally describe conditions or substances that do not cause harm. Among the provided options, "C) harmless" is the most accurate antonym because it directly signifies the absence of any harmful effects or risks.

In contrast, the other options convey meanings that align with negative consequences or misleading implications. "A) toxic" indicates something that is poisonous or harmful, which is the opposite of what we seek in an antonym of "deleterious." Similarly, "D) lethal" suggests something that can cause death, thus also representing a harmful characteristic. Lastly, "B) spurious" typically refers to something that is not genuine or is false, but it does not inherently convey harm or protectiveness, making it less relevant in a discussion about opposites of "deleterious."

In summary, "harmless" clearly stands out as the correct choice, comprehensively embodying a notion that opposes the harmful implications associated with "deleterious."