Toni has . . . . . . . . terrible Stomach ache.

Answer: Option A

The choice between "a" and "an" depends on the sound at the beginning of the word, not the letter.

The word "terrible" begins with a consonant sound (/t/), not a vowel sound.

Therefore, the correct article is "a", not "an".

"An" is only used before words that begin with vowel sounds (like "an apple", "an hour").

Hence, the correct sentence is:

Toni has a terrible stomach ache.