Meq of $H_2SO_4$ =Meq of NaOH
$N\times 5=84.6\times 2$
$N=\large\frac{84.6\times 2}{5}$
$\;\;\;\;=33.84eq/l$
Weight of $H_2SO_4$ in 1litre =$33.84\times 49$
$\Rightarrow 1.658g/ml$
$\therefore $ purity =$\large\frac{1.658}{1.8}$$\times 100=92.12\%$
Hence (c) is the correct answer.