Option D is the correct answer because “alternatively” means as an another option/choice, hence it is correct.
Option A is incorrect because “alternately” means to occur in turn, repeatedly, which means he should first travel by sea then by road, then again by sea, which is incorrect.
Option C is incorrect because “conversely” means on the contrary or instead of, hu there, the speaker gives them both the option as possible options to be done.
Option B is incorrect because “alteringly” means change in the composition of something, in a small or big significance.