18) F

1 Hydrogen atom is removed and replaced with -CONH₂ group.
Formula with 1 less hydrogen atom = CnH2n+1
When the -CONH₂ group is added,

The number of carbon atoms increases by 1
Number of Hydrogen atoms increases by 2
Oxygen atom is added
Nitrogen atom is added

New formula would be: C(n+1)H(2n+1)+2ON = C(n+1)H(2n+3)ON

Hence F is the answer.