Answer:
D Li, Cr
Step-by-step explanation:
Lithium (Li) has the electron configuration 1s22s1, so it has one electron in its outermost s orbital. Chromium (Cr) has the electron configuration [Ar]3d54s1, so it also has one electron in its outermost s orbital.
The other options are incorrect:
Calcium (Ca) has the electron configuration [Ar]4s2, so it has two electrons in its outermost s orbital.
Scandium (Sc) has the electron configuration [Ar]3d14s2, so it also has two electrons in its outermost s orbital.
Copper (Cu) has the electron configuration [Ar]3d104s1, so it has one electron in its outermost s orbital, but it also has ten electrons in its d subshell.
Helium (He) has the electron configuration 1s2, so it has two electrons in its outermost s orbital.
Therefore, the only pair in which both atoms have one electron only in an s orbital in their ground states is Li and Cr.