Models of four atoms are shown as figures a, b, c, and d. which two atoms represent isotopes of the same element?

The identity of the element is determined by the number of protons
in the nucleus of each atom. 

If two atoms have the same number of protons in their nucleii
(nucleuses) but different numbers of neutrons, then they're both
atoms of the same element, but their atomic masses are different,
and they're called isotopes of the element.

In the picture, atoms 'A' and 'B' each have 3 protons in the nucleus,
so they're both atoms of Lithium.  But the number of neutrons is
different, so 'A' and 'B' are different isotopes of Lithium.

Also in the picture, atoms 'C' and 'D' each have 4 protons in the
nucleus, so they're both atoms of Beryllium.  But the number of
neutrons is different, so 'C' and 'D' are different isotopes of Beryllium.