The conclusion that follows from the premises 'All horses are mammals' and 'No mammals are amphibians' is:

Correct option is D

Solution:

A. Some horses are amphibians. → False (As, All horses are mammals and No
mammals are amphibians → No horses are amphibians)
B .Every amphibian is a horse. → False (As, All horses are mammals and No mammals are amphibians → No horses are amphibians)
C All horses are amphibians. → False (As, All horses are mammals and No mammals are amphibians → No horses are amphibians)
D. No amphibians are horses. → True (As, All horses are mammals and No mammals are amphibians → No amphibians are horses)
Hence correct option is; No amphibians are horses.