vehicle a is about to do a u-turn by using the road that ends. who gives way?

Vehicle A, the one about to do the U‑turn using the road that ends, must give way.

Who gives way here?

In standard road rules, a driver performing a U‑turn must give way to all other vehicles and pedestrians. This applies even if they are using a terminating road (like the top of a T‑intersection) to swing around. In other words, Vehicle A has no priority: it must wait until the way is clear and allow all other traffic to proceed first.