Which of the following refers to a male merlin in falconry?

The term that specifically refers to a male merlin in falconry is "tiercel." This nomenclature comes from the fact that male raptors are typically smaller than their female counterparts, and "tiercel" specifically denotes a male hawk or falcon, including the merlin.

The name derives from the Latin word "tertius," meaning "third," reflecting the fact that male birds are generally about one-third smaller than females. This systematic naming helps falconers identify and distinguish between the sexes of raptors, which is crucial for breeding, training, and hunting purposes.

Other terms listed, while potentially relevant in the context of raptors and falconry, do not specifically denote a male merlin. For example, "jack" can refer to a male bird in general, often used informally, but it lacks the specificity of "tiercel." "Jerkin" is an informal or archaic term sometimes used for a type of young hawk or falcon, but again not specifically for male merlins. "Hawklet" refers to a young or small hawk more generally and does not denote a specific gender or species.