When flying around Europe, you will maintain contact with ATC using VHF radio calls.

In a remote area, such as the North Atlantic, VHF is not an option due to range limitations. Here, you may use HF radio. One problem with HF is the high level of background noise, which makes keeping a listening watch difficult.

So in the ocean, you are not required to listen for calls on HF, but instead the ATS unit will alert you that they wish to communicate by using SELCAL.

SELCAL stands for Selective Calling. When used, it alerts the crew by a light / noise in the cockpit (think of it as the phone ringing). When received, you pick up your headset and call in using HF where the ATS unit is waiting for you. Now, on the radio, you could receive clearances, instructions etc.

As for the "correct" answer, it's not entirely accurate to say that SELCAL is used to permit the controller to give instructions or clearance. The controller can give instructions or clearance even if the aircraft is not equipped with SELCAL. The primary purpose of SELCAL is to alert the crew that they need to make contact with ATS.