"Grandmother is writing in the book."
Translation:दादी किताब में लिख रही है ।
vinay92's explanation for a similar question on another thread...
Q: Am I crazy or should this be in the formal tone using "hain"?
A: Not necessarily. It is not unheard of to refer to your close elder relatives with the tum-level of formality as a show of endearment. This is especially true for your mother or other close female relatives.
Of course, if the sentence with हैं is not also accepted as a correct answer, you should report it.