He offered no explanation, … apology.

Analyze the question: The sentence is "He offered no explanation, ____ apology." We need an expression that means "and certainly not an apology," because an apology is an even bigger step than an explanation.

Concept: The idiom is "still less" (or "much less"). It is used after a negative statement to say that something even smaller or more unlikely is also not true.

• He didn't speak to me, still less did he help me.
• He offered no explanation, still less an apology.

Checking each option:

• A. still less an - Correct. "Still less" is the idiom, and the article "an" is needed because "apology" is a singular countable noun beginning with a vowel sound.
• B. and even less an - "Even less" is not the fixed idiom here, and adding "and" after the comma makes the structure awkward. It is not the standard form.
• C. not less the - Meaningless in this context.
• D. still less the - The idiom is right, but "the" is wrong. We mean "an apology" in general, not a specific, known apology.

Solution: "He offered no explanation, still less an apology." This means he gave no explanation, and he certainly gave no apology.

Answer: A

Tip: Remember the pattern "no/not X, still less Y" (or much less Y). Y is always the stronger or less likely thing. Also check the article: a general singular noun like "apology" takes a/an, not the.