Reaching point $M$ the bus traveled $20$ km and consumed $12$ units of electricity, So, electricity consumption per km$=\frac{12}{20}.$
From point $M$ to point $N$ distance traveled $=45-20=25,$ electricity consumed $=25-12=13.$ So, electricity consumption per km $= \frac{13}{25}.$
Likewise at point $O,$ electricity consumption per km$=\frac{20}{30}$ and at point $P$ it is $\frac{12}{25}.$ So, least electricity consumption per km is at point $P.$
Answer is D