The given conditions should be satisfied by atomic orbitals to form molecular orbitals:

$(a)$ The combining atomic orbitals must have the same or nearly the same energy. This means that in a homonuclear molecule, the $1 s$ – atomic orbital of an atom can combine with the $1s$ – atomic orbital of another atom, and not with the $2 s$ – orbital.

$(b)$ The combining atomic orbitals must have proper orientations to ensure that the overlap is maximum.

$(c)$ The extent of overlapping should be large.