We know that the side opposite to largest angle is longest side of triangle .
∠A is the largest angle and side opposite to it is BC
So BC is the longest side.
Then comes ∠C and side opposite to it is AB.
So AB is second longest side.
Then comes ∠B and side opposite to it is AC.
So it is the smallest side.
So the decreasing order of side is
BC>AB>AC
So option A is correct.