Answer:
The correct option is an atom with a partly filled energy level
Step-by-step explanation:
An atom with a partially filled energy level means the atom has a partially filled outermost shell which in turn means the atom as valence electrons. The presence of valence electrons in atoms makes them undergo chemical bonding with other atoms of the same/different elements (during chemical reactions).
Atoms with completely filled energy levels (such as the noble gases) have a completely filled outermost shell and hence do not have valence electrons which can take part in bond formation during chemical reactions and hence these group of atoms have low chemical reactivity.