As the cost of wireless service has steadily dropped over the last year and as mobile phones have become increasingly common, many people are finding that they can avoid toll charges on their home phones, using their mobile phones for making long-distance calls at night or on weekends, at a time which many wireless companies provide unlimited airtime for a small monthly fee

a) same as above
b) phones, instead using mobile phones to make long-distance calls during the night of weekends, during which
c) phones by using their mobile phones to make long-distance calls at night or on weekends, when
d) phones using mobile phones for making long-distance calls during the night or weekends, when
e) phones when using their mobile phones to make long-distance calls at night or on weekends, a time which.

The correct answer is C.
My doubt is, Kaplan says - when will always refer to a specific time period; else its use is wrong.

For eg -

Correct: It was a historic event when, in 1947, India achieved independence.
Incorrect: When the price goes up, the demand falls down.

Is it okay to use when in this context?