Hello!
"A" is used after consonants(Letters other than a,e,i,o, and u)
"An" is used after vowels(a,e,i,o,u)
After the word "a" is exciting. "Exciting" starts with a vowel(a), so therefore, it is incorrect.
If you use "an", it will make more sense.
"What a exciting summer it has been for Jaila" doesn't really make sense.
Try it with "an"
"What an exciting summer it has been for Jaila" makes sense. Therefore, it is the answer.
![\framebox{](https://img.qammunity.org/2019/formulas/english/high-school/lk4s0ncisrfjxvg1pnjferyk2epjf6bwcm.png)