What is a method for preventing general voice-quality degradation caused by too many concurent active calls?

What is a method for preventing general voice-quality degradation caused by too many concurent active calls?

A.
Low Latenxy Queuing (LLQ)

B.
compressed Real-Time Protocol (cRTP)

C.
Call Admission Control (CAC)

D.
Commited Access Rate (CAR)