Only Dev handles the master key. Choose the standard simple-present form of hold, keeping the relative clause about Dev alone: Dev is the only one of the assistants who ___ the master key.

Explication

Holds is singular because the context makes Dev alone, the phrase headed by one, the antecedent of who. Hold remains possible under a plural attachment to assistants in an unscoped sentence, but that reading contradicts the explicit premise here; the other forms do not meet the simple-present instruction.