Why was Congress concerned about maintaining the balance between free
states and slave states?

Because without a proper balance between the two, the union may not have been established in first place,

Either because one of the two blocks - the slaves states and the free states- would have been set too powerful for the other to participate in the determination of public policy. Or because the blocks would have not agreed to join the union under such circumstances in first place.

the correct answer option is: weapons.

explanation:

tensions rose as the two opposing superpowers competed for 'weapons'.

referring to the cold war (1945-1991), the two superpowers, usa and ussr, experienced a phase of struggle and conflict with each other. these two superpowers considered each other as a threat to their survival and thus opted for policies to strengthen their positions.

the two superpowers never went to war with each other directly but competed for weapons to preserve their positions.