The correct option is A 6.9cm
Given, BC = 6cm and ∠B=45∘
We know that, the construction of a triangle is not possible, if sum of two sides is less than or equal to the third side of the triangle.
i.e., AB+BC≤AC
⇒ BC≤AC−AB
⇒ 6≤AC−AB
So, if AC – AB = 6.9cm, then construction of ΔABC with given conditions is not possible.