Which of the following elements has highest electronegativity?

The electronegativity of elements decreases down a group in the periodic table. Given that all elements—beryllium, magnesium, calcium, and strontium—are alkaline earth metals in Group 2, beryllium, being highest in the group among these, exhibits the greatest electronegativity. Pauling scale values confirm this: $1.57$ for Be, $1.31$ for Mg, $1.00$ for Ca, and $0.95$ for Sr.

Final Answer: The answer is $\boxed{D}$.