Let the time taken by Partha to cover 60 km be x hours.
Narayan will cover 60 km in x-4 hours. 

Speed of Partha = $$frac{60}{x}$$
Speed of Narayan = $$frac{60}{x-4}$$

Partha reaches the mid-point of A and B two hours before Narayan reaches B.

=> $$dfrac{30}{frac{60}{x}} + 2 = dfrac{60}{frac{60}{(x-4)}}$$

$$frac{x}{2} + 2 = x-4$$
$$frac{x+4}{2}=x-4$$
$$x+4=2x-8$$
$$x=12$$

Partha will take 12 hours to cross 60 km.
=> Speed of Partha = $$frac{60}{12}=5$$ Kmph.
Therefore, option D is the right answer.