Why did president jimmy carter believe that the soviet invasion of afghanistan posed a significant threat?
the soviet invasion began to destabilize the region.
the soviet invasion was likely to expand into neighboring countries.
the soviet invasion undermined the u. s.-led alliance in the region.
the soviet invasion was likely to bring economic ruin to the region.

The answer is:

The Soviet invasion was likely to expand into neighboring countries.

By gaining more territory more and more nations were joining the URSS, the united states wanted to keep the URSS as it was and if it was possible to diminish it, at that time the whole world was divided in communists vs "the free world" and the URSS and the United States were the leaders of each side.