Given:
- Time taken to travel 8 km and back in still water is 2 hours
- Water velocity in the lake is 4 km/hr

To Find:
- Time taken for going upstream of 8 km and coming back

Assumptions:
- The boat's speed is constant in still water, unaffected by the water velocity
- The boat's speed upstream is slower than its speed downstream

Calculations:

Speed in still water:
Let x be the speed of the boat in still water.

Speed downstream = Speed in still water + Water velocity
Speed downstream = x + 4 km/hr

Speed upstream = Speed in still water - Water velocity
Speed upstream = x - 4 km/hr

Time taken to travel 8 km and back in still water:
Distance = Speed × Time

Downstream: 8 km = (x + 4) km/hr × t1
t1 = 8 / (x + 4)

Upstream: 8 km = (x - 4) km/hr × t2
t2 = 8 / (x - 4)

Total time taken = t1 + t2

Given that the total time taken is 2 hours:
2 = 8 / (x + 4) + 8 / (x - 4)

Solving the Equation:
Multiplying through by (x + 4)(x - 4) to eliminate the denominators:
2(x + 4)(x - 4) = 8(x - 4) + 8(x + 4)

Expanding and simplifying:
2x^2 - 16 = 8x - 32 + 8x + 32
2x^2 - 16 = 16x

Rearranging and simplifying:
2x^2 - 16x - 16 = 0

Dividing through by 2 to simplify the equation:
x^2 - 8x - 8 = 0

Using the quadratic formula to solve for x:
x = [-(-8) ± √((-8)^2 - 4(1)(-8)))] / (2(1))

Simplifying:
x = [8 ± √(64 + 32)] / 2
x = [8 ± √96] / 2
x = [8 ± 4√6] / 2
x = 4 ± 2√6

Time taken for going upstream of 8 km and coming back:
The time taken for going upstream of 8 km and coming back is equal to the total time taken when the boat's speed in still water is 4 + 2√6 km/hr and 4 - 2√6 km/hr, respectively.

Therefore, the time taken for going upstream of 8 km and coming back is approximately 120 minutes (D) when rounded to the nearest whole number.