I found this question in NCERT Exemplar and can't solve it . Please help!!

If x=a ,then , which of the following will not always be true for any integer k.

a) k*a = k*x
b) k-a = k-x
c) k+a = k+x
d) a/k = x/k

I thought it should be none of these as all these r always true. But in the solutions it is given that d) can not always be true. I can't understand the logic behind that.
Further,I asked one of my teachers and she said that the logic is that if k = 0 then it will become undefined and undefined cannot be equal to undefined.
Is this true? Please answer fast.