Oleum is basically $H_2S_2O_7$, a mixture of $(H_2SO_4+2SO_3 )$
Meq. of $H_2SO_4$ +Meq. of $SO_3$=Meq. of $NaOH$
Suppose the mass of $SO_3$ in Oleum =w
Therefore the mass of $H_2SO_4=0.5\;w$
Eq.wt of $H_2SO_4=49$
and Eq.wt of $SO_3=\large\frac{80}{2}$$=40$
$\large\frac{(0.5 -w)}{49}$$ \times 1000 + \large\frac{w}{40}$$ \times 1000 =26.7 \times 0.4$
$w=0.103$
$\therefore \%$ of $SO_3=\large\frac{0.103}{0.5} $$\times 100=20.6 \%$
Hence c is the correct answer.