Which does ATC not require you to read back?

The AIM's readback list

AIM 4-4-7 asks pilots to read back the parts of ATC clearances and instructions that contain altitude assignments, vectors or runway assignments, as a means of mutual verification. A heading is a vector, so "turn right heading 180" gets read back. An altimeter setting isn't on the list, so a simple acknowledgment meets the expectation.

Hold short instructions

Hold short instructions get read back too. AIM 4-3-18 tells pilots to read back any hold short instruction, and the controllers' rulebook, FAA JO 7110.65 (3-7-2), tells them to ask for the readback if you don't give one.

Pilot's discretion

"Descend at pilot's discretion, maintain 3000" sounds optional, but the discretion only covers when you start down and how fast you descend (AIM 4-4-10). The 3000 is still an altitude assignment, and altitude assignments get read back.

Read it back anyway

Reading back the altimeter setting is still a good habit, and plenty of CFIs teach it. Outside the U.S., ICAO rules and countries like the UK do require it, which is why this quiz is specifically about the AIM and JO 7110.65.