ジョンさんは 口が かるい。
a. John is good at kissing.
b. John is lighthearted.
c. John is tight-lipped.
d. John is unable to keep secret.
The correct answer is d. 口が かるい means “to have a loose tongue” or “to have a big mouth.”
Other common expressions using 口 are:
口がうまい = to have a honeyed tongue
口がおもい = to be slow to speak, to be in-communicative
口がかたい = to be tight-lipped
口がこえる = to have a discriminating palate
口がすべる = to make a slip of the tongue
口がわるい = to be foulmouthed, to have a sharp tongue
口に出(だ)す = to express, to put into words.
口を出(だ)す = to interrupt, to intrude