The formula of functional group for aldehyde is:

As per Table 4.3 of the NCERT chapter 'Carbon and its Compounds', the functional groups containing the oxygen heteroatom are: Alcohol $-\mathrm{OH}$, Aldehyde $-\mathrm{CHO}$ (shown in the book as a carbon double-bonded to oxygen and single-bonded to hydrogen), Ketone $-\mathrm{CO}-$ (a carbon double-bonded to oxygen, attached on both sides to carbon chains), and Carboxylic acid $-\mathrm{COOH}$.

Option B ($-\mathrm{CHO}$) is correct — this is the standard condensed way of writing the aldehyde functional group given in the textbook, where the terminal carbon is doubly bonded to oxygen and singly bonded to one hydrogen atom, e.g. in hexanal, $\mathrm{CH_3-CH_2-CH_2-CH_2-CH_2-CHO}$.

Option A ($-\mathrm{COOH}$) is wrong — this is the carboxylic acid functional group (e.g. ethanoic acid), not aldehyde.

Option C ($-\mathrm{C}-$) is wrong — a bare $-\mathrm{C}-$ with single bonds does not represent any functional group by itself; it is an incomplete/generic depiction and does not show the carbon-oxygen double bond that characterises aldehydes.

Option D ($-\mathrm{OH}\ O$) is wrong — this garbled combination does not correspond to any single functional group in Table 4.3; $-\mathrm{OH}$ alone is the alcohol group, not aldehyde.