Need grammar help??????

Jonathan is close, but neither answer is completely right. Here's the deal. The "has developed" indeed refers to a skill he's still using. This is "present perfect" tense, and it describes an action which started in the past and continues into the present, for example "She has waited in that doctor's office for twenty minutes." The meaning is that she's still there. The "had developed" refers to an action which happened even earlier than another one in the past. It's "past perfect," and is used to compare two actions in the past and stipulate which one came first. For example, "She had waited twenty minutes in the doctor's office when she fell asleep." There are two actions in that sentence, the "wait" and the "fall asleep." Use of past perfect shows that the "wait" happened first. Another example is "He had eaten a candy bar when we offered him a burrito." Because "had eaten" is the earlier action, I know that the taste of candy was still in his mouth when we showed him the burrito. Your question was "Which one is correct?" and as you can see, they both are, depending on what you mean.

It depends if he's still using the skill. If he is: "The marketing skill that he HAS developed over the years at ABC will come in handy at his new advertising job at XYZ." If he isn't: The marketing skill that he HAD developed over the years at ABC is unusable in his new job as a janitor."

Depends on if he still has the marketing skill, one is pastense the other is pretense