Which of the following graphs represent a first order reaction ( $\mathrm{a}=$ initial concentration of reactant, x= concentration of reactant consumed, $\mathrm{t}=$ time)

For a first order reaction
$\text{ Rate }=-\frac{\mathrm{\Delta }\mid \mathrm{A}]}{\mathrm{\Delta l}}=\mathrm{k}\left[\mathrm{A}\right]$
It is a straight line plot with negative slope when concentration of reactant is considered as in (i) and half-life for first order is independent from concentration, as in plot (ii).