Concept:After a negative statement, the idiomatic phrase "still less" introduces something even more unlikely or impossible.
Explanation:The first part of the sentence must contain a negative idea, such as "not", "never", "hardly", or "cannot".
When that happens, "still less" is used to add a second noun phrase that is even less true or possible than the first.
The noun after "still less" is usually general, not specific, so it requires an indefinite article: "a" or "an".
Since the word following the blank begins with a vowel sound, "an" is the correct article.
"The" would be incorrect because it refers to a particular person or thing, which is not the meaning here.
Option B, "and even less an", is not as idiomatic as the standard expression "still less".
Answer:A. still less an