Which of the following layers of TCP/IP stacks is the most difficult to secure?

A.
Physical

B.
Network

C.
Transport

D.
Application

Explanation:
Securing the application layer is difficult since the application code on that layer is more diverse and written by many different entities. There is no security standard, and too many different developers are involved.
Incorrect Answers:
A, B, C: are lower layers within the OSI model. These layers are more standardized, and there are too not many variations. Outside of vendors, there are very few entities that will write their own interfaces on those levels. This makes it easier to standardize, and secure.