First of all , we should know what a bounded lattice is :
A bounded lattice is a lattice in which we have two extreme elements known as least and greatest such that all other elements lie in in between .. i.e. for any other point 'x' in a lattice :
a) LUB of x and the topmost element(represented by 1) will give the greatest element..
b) GLB of x and bottommost element(represented by 0) will give the least element.
Now two elements x and y are complementary iff :
a) GLB of x and y leads to 0(least element)
b) LUB of x and y leads to 1(greatest element)
Now if we consider least and greatest element itself ..
a) LUB of least element and greatest element : Greatest element
b) GLB of least element and greatest element : Least element
And for a bounded lattice , both greatest and least element will be present for sure..
Thus greatest element and least element are complementary to each other and hence the statement is true.