The main purpose of stalin s five-year plans was to
a. turn the soviet union into a military power.
b. increase farm production.
c. produce more consumer goods.
d. turn the soviet union into a modern industrial power.

D. Turn the Soviet Union into a modern industrial power.

The thing is that every 5 years such an economic plan was reviewed and replanned because the priorities and conditions for industrialization usually changed in that time.

The main purpose of Stalin's five-year plans was to turn the Soviet Union into a modern industrial power - D.

He also had good reason why he wanted this. The Soviet Union in his times was a very badly run country which wa somehow lost in time. His reforms were aimed at tackling this.