His interest in the study of human behaviour is indeed very . . . . . .

The correct answer is E. deep.

The other options are not as accurate in describing the level of interest in the study of human behavior.

• Strong: This could refer to the intensity of the interest, but it does not necessarily indicate that the interest is deep.
• Large: This could refer to the scope of the interest, but it does not necessarily indicate that the interest is deep.
• Broad: This could refer to the range of topics that the person is interested in, but it does not necessarily indicate that the interest is deep.
• Vast: This could refer to the amount of information that the person knows about human behavior, but it does not necessarily indicate that the person understands it deeply.

Deep interest implies that the person has a thorough understanding of the subject matter and is able to think critically about it. It also implies that the person is passionate about the subject and is constantly seeking out new information and insights.