Imine formation is an acid-catalyzed process, however the procedure performed in this does not require the addition of an external acid. Suggest a viable source of acid in the reaction depicted.

See explaination

Explanation:

Reaction between p- toluidine and o- vaniline to form imine.This reaction does occur in the presence of a acidic source.

Here the -OH group in the o- vaniline is quite acidic so it acts a internal acidic catalyst. Hence the reaction occurred in the absence of any external acidic source.The reaction mechanism is given at the attached file.

please kindly see attachment.

1 a

2 d

3 a

4 c

explanation: