(A) 6.9 cm
Given, BC = 6cm and ∠B=45∘
We know that, when the sum of any two sides of a triangle is greater than the third side only then the construction of that particular triangle is possible.
Now lets take any pair of sides,
i.e., AC+BC>AB
⇒ BC>AB−AC
⇒ 6>AB−AC
So, if AB – AC = 6.9cm, then construction of ΔABC with given conditions will not possible.