Which of the following was an effect of stalin’s five-year plans?
a. the soviet standard of living increased greatly.
b. agricultural output increased.
c. the soviet union became a major military power.
d. heavy industry expanded.

Stalin's five-year plans were actually very complex and more than one answer here is correct. His plans were very daring for its times and he greatly expanded the heavy industry of the Soviet Union. For that reason D is correct; Heavy industry expanded.

What also happened though was that the agricultural output increased, as he wanted for his people to have more food (and not starve basically), so this was another effect of his Five-year plans; B - Agricultural output increased.