Answer:
a / the
both are correct
Step-by-step explanation:
We use the article 'a' when we refer to something but the object is not specific & 'the' is used when in the given context we know the particular hospital that the grandmother is in. Both are correct & can be used here.
'An' is wrong as 'an' is usually used only in front of words starting with the vowels: a, e, i, o & u.