Pretty sure it's because "engineer" has a masculine ending here, and with any of the other options (all female relatives) it would have changed to have the female ending to match the gender of person.
Masculine : جدي مهندس
Feminine : جدتي مهندسة
Maybe I'm missing something but are we to infer that the answer is grandfather because "engineering" is a mans job? Or is it that the word engineering is written in a masculine way?
Haha. Its because has masculine declination