Question

an effective method for immobilizing an injured shoulder is to—
use a rigid splint.
all of the above
use the other shoulder as an anatomic splint.
hold the arm against the chest.

Explanation:

When immobilizing an injured shoulder, using a rigid splint can provide external support. Using the other shoulder as an anatomic splint (by taping or securing the injured arm to the body in a way that the uninjured side helps hold it in place) and holding the arm against the chest (which can be done with slings or other methods) are also valid immobilization techniques. So all the mentioned methods are effective.

Answer:

All of the above