The metallic character of elements decreases from left to right across a period. Thus, the metallic character of Mg is more than that of Al. The metallic character of elements increases down a group. Thus, the metallic character of Al is more than that of B. Considering the above statements, we get K > Mg and the only relation out of the given relations which holds the above condition is - (d) K > Mg > Al > B.

Hence, the correct order of metallic character of the elements B, Al, Mg, and K is – (d) K > Mg > Al > B.