Answer:
B.) a methyl group
Step-by-step explanation:
A.) is incorrect because an amino group has a central nitrogen atom bonded to two hydrogen atoms.
B.) is correct because methyl groups consist of one central carbon bonded to three hydrogen atoms.
C.) is incorrect because a phosphate group contains a phosphorus central atom and four surrounding oxygen atoms. It should be noted that one of the oxygens is double bonded to the phosphorus, while the other three oxygens are single bonded to the phosphorus.
D.) is incorrect because carbonyl groups consist of a central carbon atom double bonded to an oxygen atoms. The carbon is also bonded to two R groups (normally some type of carbon chain).