For an element say x, to be a complement of 'a'.
The least upper bound of 'a' and 'x' should be the upper bound of the lattice which is 'f' here.
The greatest lower bound of 'a' and 'x' should be the lower bound of the lattice which is 'j' here.
Considering these two properties, b, c, d, e is not the complement of 'a' because the least upper bound between 'a' and any one of these elements is 'h'.
Only i is the complement of 'a'.
Ans: B.