Answer:
A. U-boats
Step-by-step explanation:
The reason I believe the answer is A. is because, it is asking for important warfare that was used in the SEA. Bayonets are a knife, dagger, sword, or spike-shaped weapon designed to fit on the end of the muzzle of a rifle, musket or similar firearm. An ironclad is a steam-propelled warship protected by iron or steel armor plates. Poison gas is of course, deadly gas. "U-boat, German U-boot, abbreviation of Unterseeboot, (“undersea boat”), a German submarine."
The U-boat is the only answer that can make sense to be used for sea.