The Kirby ability that allows him to turn into a rock is the Stone ability. When Kirby uses Stone, he becomes a heavy, rock-like form that can withstand or avoid many attacks and crush enemies from above.
Stone is one of Kirby's Copy Abilities, a system introduced in Kirby's Adventure in 1993. Kirby usually obtains a Copy Ability by inhaling a suitable enemy and swallowing it. For Stone, the source enemies commonly include Rocky, a small rock-like foe associated with the ability in several Kirby games.
The ability is useful because Kirby can become temporarily invulnerable while in stone form in many games. He can drop quickly, smash through certain obstacles, and damage enemies by landing on them. The exact moves and appearance vary across installments, but the central idea remains the same: Kirby trades mobility for weight, defense, and crushing power.
Ground, Heavy, and Metal are plausible-sounding alternatives, but they are not the standard name of this Copy Ability. “Stone” is therefore the single canonical answer expected by a fair quizmaster.