Reaction of cyclohexanone with dimethylamine in the presence of catalytic amount of an acid forms a compound if water during the reaction is continuously removed. The compound formed is generally known as

Cyclohexanone is a ketone; dimethylamine, $$\left(CH_3\right)_2NH$$, is a secondary amine.

General carbonyl-amine condensations proceed as follows:

• With a primary amine ($$RNH_2$$) the carbonyl first gives a carbinolamine, then—after elimination of water—an $$C{=}\!N$$ double bond (imine/Schiff base) is produced.
• With a secondary amine ($$R_2NH$$) the carbinolamine formed initially cannot lose a second N-H proton (none is present). Instead, it eliminates the α-hydrogen of the carbonyl compound, giving a C=C-N skeleton known as an enamine.

The overall transformation for cyclohexanone is:

$$\text{cyclohexanone} + \left(CH_3\right)_2NH \;\xrightarrow[\text{-H}_2\text{O}]{\text{acid cat.}}\; \text{enamine} + H_2O$$

Continuous removal of water shifts the equilibrium to the product side (Le Chatelier’s principle) and ensures good yield of the enamine.

Thus the compound obtained is an enamine.

Option B which is: an enamine