Define functional group. Write any four functional groups with their formula.

Sometimes, in organic chemistry, we observe compounds containing atoms other than carbon and hydrogen also as there is a chance for other chemical substituents to get linked with the given organic compound via the bonding site. 

These chemical substituents bring new properties to the organic compound and aid in the forming of various other substituted organic compounds. 

Such groups that are attached to the main carbon chain and impart their characteristic properties are known as functional groups.

The functional groups are chemical substituents and some of the common functional groups are as follows:
Alcohol - $\mathrm{R}-\mathrm{OH}$

Acid - $\mathrm{R}-\mathrm{COOH}$

Aldehyde - $\mathrm{R}-\mathrm{CHO}$

Ketone - $\mathrm{RCOOR}$

Hence option A is correct.