Because the line BC is parallel to the proposed movement to the vertex C, it means that this movement will not interfer in the position of the line BC in relation to the line AB, and for this reason the angle B will be the one that will not change in this movement.
Now, we know also that the sum of internal angles in the triangle must be always constant, if the angle C starts to decrease, because of the increasing of BC, and the angle B remains unchanged, the angle A must also change to maintain the sum not changed. Because the angle C decreases, the angle A must decrease in the same proportion.
From the solution developed above, we are able to conclude that the correct answer is:
A. by showing that as BC increases, the angle A increases.