In UML sequence diagrams, a signal is typically represented as an asynchronous message without a return value. Asynchronous messages are often denoted with an open arrowhead, and they can represent the sending of a signal.

Looking at the diagram provided, 'p' is sent from 'v2[C]' to 'v3[B]' with an open arrowhead, which indicates that it is an asynchronous message. This fits the common representation of a signal, as it does not expect a response.

'A', 'B', and 'C' do not represent signals in the context of a sequence diagram: A) 'v' is not shown in the diagram; it might refer to a lifeline, which is not a signal. B) 'B' refers to a class or object type and not a message or signal. C) 'm' is represented as a synchronous message (as it has a return arrow), which is not typical for signals.

Therefore, the correct answer is:

D. p