Every hour, the virus increases 16 times.
Due to the anti-agent presence in the system, only 1/2 survive by 16 * 1/2 times = 8 times per hour.
Starting with x virus and after 8 hours, there are;
$$2^{40}\ virus$$
$$x\cdot8^8=2^{40}$$
$$x\cdot\left(2^3\right)^8=2^{40}$$
$$x\cdot2^{24}=2^{40}$$
$$Divide\ through\ by\ 2^{24},\ and\ apply\ the\ law\ of\ indices$$
$$We\ have,\ x=2^{40-24}=2^{16}$$

Therefore, the correct answer is option A