The condition necessary for stable structure of solids is _________________.

The correct option is B both attractive and repulsive forces must act on them.
The condition necessary for stable structure of solids is both attractive and repulsive forces must act in them. When the attractive and repulsive forces are balanced, the atoms/molecules are held in there position. When the attractive forces are strong, the atoms/molecules are pulled towards each other. When the repulsive forces are strong, the atoms/molecules are pulled away from each other. Thus, the proper balance between the attractive and repulsive forces will ensure the stable structure of solids.