ANSWER
OPTION B
Step-by-step explanation:
Compounds are pure substances that are composed of atoms of two or more different elements that are chemically combined together.
Looking at the images provided in the question, you will see that the atoms in model X are chemically combined together due to one of the atoms releasing electrons and the other atom receiving it just to complete their octet structure. Hence, Mode X is a compound because two different atoms are chemically combined together.
The correct option is B