Correct Answer - C
If the speed downstream is a kmph and the speed upstream is b kmph
then
Speed of the stream = `(1)/(2)` (a-b) kmph
Explanation
Speed downstream a = 12 kmph
Speed upstream b = 8 kmph
Speed of the stream = `(1)/(2) (a-b) = (1)/(2) (12-8)`
= `(4)/(2) = 2` kmph