The cost of a given amount of computing power has .
depended on the current economy
stagnated over the last five decades
decreased drastically over the last five decades
decreased slightly over the last five decades

C. Decreased drastically over the last five decades.

Explanation:

The cost of a given amount of computing power has decreased drastically over the last five decades. This can be proven beyond reasonable doubts with the invention of smartphones. Overtime, smartphones have become less expensive for a certain amount of computing power and easy to use.

Contrary to what was obtainable in the past where we have the large-scale Vacuum tube UNIVAC computers, ENIAC and the IBM series for computing. They weren't only big in size, they had a very small processing speed and very expensive too.

Smartphones has by far outlived the above-mentioned devices in all ramifications owing to the fact that it's relatively cheap, small in size, high processor speed and larger storage space.

make sure each item is its own paragraph.