high Fowler’s with the unaffected arm elevated on a pillow
flat with the client positioned on the affected side or back
semi-Fowler’s with the affected arm supported on a pillow
reverse Trendelenburg’s with the arm on the affected side supported on a pillow
Number 3 is correct.
Rationale: Following a mastectomy, the client should be positioned with the head of the bed at 30 degrees or more, with a pillow supporting the arm on the affected side to allow lymphatic fluid return. The other options describe either incorrect bed positioning or incorrect placement of the pillow.