Step 1: Identify what the question is testing.
This is a classic double-negative error, one of the most commonly tested error types in verbal ability sections.
Step 2: Recall the rule.
Words like "hardly", "scarcely" and "barely" already carry a negative meaning by themselves. They should never be paired with another negative word such as "not" or a contraction like "hadn't", "didn't" or "couldn't". Using both together creates a double negative, which is grammatically incorrect in standard English.
Step 3: Apply the rule.
The sentence reads "The sun hadn't hardly set...", combining "hadn't" (a negative) with "hardly" (another negative). This should instead be "The sun had hardly set when the mosquitoes began to sting...", using the positive form "had" together with "hardly".
Step 4: Locate the error.
The faulty phrase "hadn't hardly" occurs entirely within segment (a), "The sun hadn't hardly".
Step 5: Confirm the rest.
Segments (b), (c) and (d) read smoothly and contain no grammatical issues, so the error is confined to option (a).