Most realistic growth curve 

The resources for the growth of most animal population are finite and become limiting sooner or later, thus the logistic growth model is considered more realistic. The equation for model gives a sigmoid shaped graph when plotted against the N in relation to time.  

The equation is :

$\frac{dN}{dT}$ = $rN$ $\frac{(K-N)}{K}$ 

where, 

N is Population density at time t

r is the intrinsic rate of natural increase 

K is carrying capacity.

So, the correct answer to the question is 'Sigmoid'.