Explanation

(c) $\rho = \frac{PM}{RT}$
Density $\rho$ remains constant when $P/T$ or volume remains constant.
In graph (i) Pressure is increasing at constant temperature hence volume is decreasing so density is increasing. Graphs (ii) and (iii) volume is increasing hence, density is decreasing. Note that volume would had been constant in case the straight line the graph (iii) had passed through origin