Final answer:
DEET contains an ester functional group, characterized by a -CO₂R structure, differentiating it from aldehydes, ketones, and amides.
Step-by-step explanation:
The functional group present in DEET is an ester. Esters are characterized by the presence of a -CO₂R group where the R represents an organic substituent. DEET, also known as N,N-diethyl-meta-toluamide, includes this functional group, which is pivotal in its activity as an insect repellent. In comparison, aldehydes contain a -CHO group, ketones contain a -CO- group, and amides contain a -CONH2 or -CONR2 group, where R represents a hydrocarbon group.