(d) 80 km
Let the distance between the two parts be x km.
Then, speed downstream =\(\frac{x}{4}\) km/hr
and speed upstream = \(\frac{x}{5}\) km/hr
⇒ Speed of stream = \(\frac12\big(\frac{x}4-\frac{x}5\big)\)
Given, \(\frac12\big(\frac{x}4-\frac{x}5\big)\) = 2 ⇒ \(\frac{x}4-\frac{x}5=4\)
⇒ \(\frac{x}{20}=4\) ⇒ x = 80 km.