In CSMA/CD, for success, only 1 station should transmit while others shouldn't.
P(success)=nC1*p * (1-p)n-1 Binomial thm.
For max P(success), differentiate and equate to zero(line in limits to get maxima and minima).
We get P(max)=1/e
Number of times we need to try before getting 1st success= $\frac{1}{p(max)}$ = $\frac{1}{\frac{1}{e}}$=e
Efficiency=$\frac{transmission time(Tt)}{number of collision(C)*2*propagation time(Tp)+Tt+Tp}$
Here number of times we need to try=e, ie C=e.
Put a=$\frac{Tt}{Tp}$ and divide by Tt, we get: $\frac{1}{e*2a+1+a}$
Put e=2.72, efficiency = $\frac{1}{1+6.44a}$