B.C.
It's nothing new.
When I was growing up we all had to have a spoonful of cod liver oil every day because the high levels of vitamin D in cod liver oil have been shown to prevent rickets and other symptoms of vitamin D deficiency.
She's growing and so are her bones and joints.
Vitamin D is good for her bones - why would you not want her to have a nutrient which is good for her?
Vitamin D requirements for infants can't be met by human breast milk alone.
Granted too much of a good thing can be bad - so don't give her too much, - following the dosage of the drops would be the right amount.