The correct answer is option-1, Potassium. In a nuclear reactor, liquid sodium is used as a coolant to transfer heat away from the reactor core. To improve its heat transfer properties, sodium is often alloyed with other metals. One such metal is Potassium.

Option-2, Calcium, is not the correct answer. Calcium is not typically alloyed with sodium for heat transfer in a nuclear reactor.

Option-3, Magnesium, is also not the correct answer. While Magnesium is a commonly used metal in alloys, it is not specifically alloyed with sodium for heat transfer in a nuclear reactor.

Option-4, Strontium, is not the correct answer either. Strontium is not commonly alloyed with sodium for heat transfer in a nuclear reactor.

Therefore, the correct answer is indeed option-1, Potassium. It is alloyed with sodium to enhance its heat transfer properties in a nuclear reactor.