Which description of RTP timestamps or sequence numbers is true? 

Answer options:

A. The sequence number is used to detect losses.
B. Timestamps increase by the time "carrying" by a packet.
C. Sequence numbers increase by four for each RTP packet transmitted.
D. The timestamp is used to place the incoming audio and video packets in the correct timing order (playout delay compensation).

D

Reference: https://www.cs.columbia.edu/~hgs/rtp/faq.html