Explanation:

Let the speed of boat in still water = x km/h

speed of stream = 3 km/h

= speed of boat in upstream = x - 3 km/h

= speed of boat in downstream = x + 3 km/h

Distance traveled in downstream = Distance traveled in upstream = 2 km

Total Time taken = 30 minutes = 0.5 hr

In upstream,

time = distance/speed

= t1 = 2/(x-3)

In downstream,

time = distance/speed

= t2 = 2/(x+3)

total time = 0.5

= t1 + t2 = 0.5

= 2/(x-3) + 2/(x+3) = 0.5

= (2x + 6 + 2x - 6)/(x²-9) = 0.5

= 4x/x²-9 = 0.5

= x² - 9 = 8x

= x² - 8x - 9 = 0

= x² +x - 9x - 9 = 0

= (x+1)(x-9) = 0

= x = -1 or x = 9

= x = 9 as speed can not be -ve

Hence, option 2 is the correct answer.