The correct option is B – CHO.
The functional group for aldehyde is –CHO.
Source: Carbon and its Compounds, Table 4.3
From Table 4.3, the functional group for aldehyde contains a carbon atom double-bonded to oxygen and single-bonded to hydrogen, written as –CHO. Examiners expect the exact formula; –COOH is carboxylic acid and –OH is alcohol — do not confuse them.