I think “Grandmother cooks a meal” or “prepares a meal” should also be accepted as correct answers. Right now that doesn’t seem to be the case. It’s a bit more natural sounding than the current translation I believe.
This sentence is wrong. Native korean here. 진지 is not used correctly. 할머님께서 밥을 짓습니다 would be more natural and correct. 밥 can mean rice or meal. 짓습니다 is only used for rice but can mean prepare depending on context.
I think 하세요 is a simple statement. 하셔요 would be used if I were to have a conversation with another person younger than grandma and I wanted to use the honorific form to talk about her. Dont know the technicalities, just my gut feeling