Final answer:
The sea anchor on a one-man raft is used to stabilize the raft in rough waters and prevent it from drifting, ensuring safety and control. So, the correct answer is 1) Stabilize the raft in rough waters.
Step-by-step explanation:
The purpose of securing the sea anchor on a one-man raft is primarily to stabilize the raft in rough waters and to prevent the raft from drifting. While selecting a raft based on color or size may be of interest to some, the most important feature of any raft, especially a one-man raft used in potentially challenging conditions, is its ability to provide safe and stable transport. A sea anchor, which acts as a drag device, reduces the speed at which the raft drifts with currents or wind, offering a greater degree of control and safety to its occupant.
Thus, the correct answer is 1) Stabilize the raft in rough waters.