Correct option is A. $$\dfrac{2}{ln(2)}M\Omega$$
$$q=q_0e^{t/\tau}$$ when energy is $$50\%$$
then $$q=\dfrac{q_0}{\sqrt{2}}$$
$$\dfrac{q_0}{\sqrt{2}}=q_0e^{-t/\tau}$$
$$e^{t/\tau}=\sqrt{2}$$
$$\dfrac{t}{\tau}=ln(\sqrt{2})$$
$$\tau =\dfrac{t}{ln(\sqrt{2})}$$
$$R_c=\dfrac{1}{ln\sqrt{2})}$$
$$R=\dfrac{1}{Cln(\sqrt{2})}=\dfrac{1}{10^{-6}ln(\sqrt{2})}=\dfrac{10^6}{ln(\sqrt{2})}=\dfrac{2}{ln(2)}=M\Omega$$.