In the following question, the sentence given with blank to be filled in with…

Choosing the right verb form means checking two things together: does the form AGREE with the subject (a third-person singular subject like ‘a ruthless serial killer’ needs an ‘-s’ ending in the simple present, e.g., lurks, dances, smiles), and does the TENSE fit the situation the sentence describes. An adverb such as ‘still’ usually signals an unbroken state — but that state can be described either in the present (‘he still lurks’) or, within a past-tense narrative, in the simple past (‘he still lurked at that point’); either is grammatical so long as subject–verb agreement holds.

Applying this to the sentence, check each candidate verb for both agreement and sense:

• Dance — semantically wrong: fleeing, hiding, and preparing to strike is not describable as dancing, whatever the tense.

• Lurked — a complete, correctly agreeing past-tense form that matches the sense of hiding and waiting to ambush; produces a fully grammatical sentence.

• Lurk — the bare/base form. Paired with the third-person singular subject in a present-tense reading it fails subject–verb agreement (the present form would need the ‘-s’: lurks), so on its own it does not complete the sentence correctly.

• Smile — semantically wrong for the same reason as Dance: it does not convey concealment or menace.

Checking the two form-correct-looking words again: only one of them stays grammatical without needing any change — the bare form fails agreement outright, while the past-tense form needs no correction and still carries the intended sense of an ongoing, unresolved threat.

Answer: Lurked — it is the only offered word that is both grammatically complete for this subject and semantically consistent with a serial killer hiding in the woods.