What was the major motivator of the arms race during the cold war?
a. both the soviet union and the united states worried that the other
would build up a larger nuclear arsenal.
o
b. the cuban missile crisis demonstrated the importance of nuclear
missiles to all countries' military strategies.
o
c. international proxy wars became so frequent that nearly all
countries built up massive military forces.
o
d. the soviet invasion of afghanistan forced the united states to
defend the middle east as well as asia and africa.

The right answer here is:

A. Both the Soviet Union and the United States worried that the other

would build up a larger nuclear arsenal.

Explanation:

During the two-superpower world competition that was the Cold War, each side always worried that the other side would enjoy military superiority, especially nuclear superiority. In a zero-sum world system, this was considered a serious, dangerous disadvantage, and in this situation, the country that had the edge could be tempted to use the nuclear  weapons first. The arms race followed a strategic reasoning called MAD, or Mutual Assured Destruction. Actually, the posession of nuclear missiles and bombs were rather intented as a tool of deterrence.

not to sure sdi, strategic defense initiative

it foreshadows the theme of the paragraph and hints the topic that will be discussed in the paragraph!