Which undesired effect of increasing the jitter compensation buffer is true?

The jitter buffer compensates for jitter at the receiver but does not reduce the actual jitter present in the network; transport jitter (variation in packet inter-arrival time) remains unchanged regardless of buffer size.

Increasing the buffer size does not cause network jitter to increase - jitter is a function of queuing and routing in the network, not of the receiver-side buffer configuration.

A jitter buffer holds arriving packets temporarily and releases them at a steady rate to compensate for variable network arrival times. Increasing the buffer depth means each packet waits longer before being forwarded to the decoder, which directly increases the end-to-end transport delay. For delay-sensitive real-time applications such as VoIP, this additional buffering delay can exceed the acceptable one-way latency threshold (typically 150 ms), causing echo, unnatural conversation gaps, and perceived quality degradation.

Increasing the jitter buffer size increases end-to-end delay (not decreases it), because larger buffers impose a longer waiting period before packets are released for playback, which worsens rather than improves delay-sensitive quality.