Answer

(ii) Two times

Explanation

In pure ALOHA, a frame can be transmitted at any time. Suppose a frame takes time $T$ to transmit.

For another frame to avoid collision with it, another transmission must not begin during the interval from $T$ before the frame starts until $T$ after the frame starts.

Therefore the vulnerable period is:

$T_v = 2T$

So the vulnerable time is twice the frame transmission time.

This is also why the maximum theoretical throughput of pure ALOHA is approximately $18.4\%$.

Final answer: (ii) Two times.