Say the word slowly in parts to check the spelling: "Al" plus "mighty". There is only one "l" at the start and the middle keeps the "gh" from the root word "mighty".
Option B fails because it doubles the "l" to "Allmighty", which is not how the word is built. Option C fails because it removes the "gh", turning it into "Almity", a spelling that does not exist in English.
Both option A and option D are printed as "Almighty" in this question, and both show the correct spelling. Since the answer key names option A specifically, that is the one marked as the accepted answer here.
The correctly spelled word is "Almighty", matching option A.