Why Is It That a Specific Feature of an Organism

Why is it that a specific feature of an organism (such as an eye) may be well-suited to its environment but not an "optimal" adaptation?

A) Evolution is constrained by the traits passed along by previous generations
B) Evolution can only act on the parts of an organism that come into direct contact with the environment
C) Evolution is not capable of producing structures that have clear design principles
D) all of the above