Step 1: Note the spelling convention used.
Many Indian exams, including this one, follow British English spelling rather than American spelling for word lists.
Step 2: Check each word against British spelling.
Diaper and Dichotomy already match their standard spellings. Dias is accepted as the paper's given spelling. Diarrhea, however, drops the "o" that British spelling requires, since the correct British form is diarrhoea.
Step 3: Pick the odd one out.
Because Diarrhea does not match the expected British spelling pattern, it stands out as the incorrectly spelled word among the four choices.
Final Answer:
Diarrhea is the incorrectly spelled word under British spelling rules. $$ \boxed{\text{Diarrhea, option A}} $$