(C) Exothermic, but the heat evolved is not sufficient for the hydrogen to catch fire.
Calcium reacts with cold water more vigorously than magnesium but less violently than sodium/potassium. The reaction is exothermic and produces hydrogen gas, but the heat released is not enough to ignite the hydrogen. (Sodium/potassium reactions are so violent that hydrogen catches fire.) This is a standard reactivity series application tested in CBSE MCQs.