Final answer:
The RTP header compression can reduce the Layer 3 and Layer 4 headers significantly, typically to 2 to 4 bytes, but none of the provided options match this range.
Step-by-step explanation:
The question pertains to RTP (Real-Time Protocol) header compression, a technique used in packet-based networks to reduce the size of the packet headers to increase bandwidth efficiency on slow links. When RTP header compression is applied, the combined Layer 3 (IP) and Layer 4 (TCP/UDP) headers can indeed be compressed significantly.
The correct size that the headers can be compressed down to in this scenario is 2 to 4 bytes depending on the compression algorithm used and the context of the connection which includes signaling for the establishment, maintenance, and termination of the compression.
However, none of the options provided in the question (12 bytes, 16 bytes, 20 bytes, or 24 bytes) correctly represents this usual range for header compression. While these values may be achieved with less aggressive compression schemes, they do not typically represent the extent to which RTP headers can be reduced.