What information does the nociceptor relay to the brain about stimuli below threshold?
Short answer
D.

Background
Nociceptors are free nerve endings, and usually consist of bundles of unmyelinated fibers (Messlinger, 1997).

So nociceptors are basically axonal fibers. Below threshold, activation of these fibers do not cause action potentials to occur. Since action potentials are, basically, all-or-nothing events no signals will be sent to the brain. Hence, the brain will not receive information on either duration (A), or location (B) of the pain stimulus. The membrane potential (C) is a non-sense answer. Membrane potential, although it may affect receptor response properties, is never the primary message of a receptor signal. It is about the sensation of physical stimuli (in this case painful mechanical, heat or chemical stimuli).

So the answer is -D- as we have disputed A, B and C.   

Note
When receptors convey graded potentials, the term 'below-threshold' doesn't really apply anymore. Graded potentials can, in principle, convey the smallest stimulus difference as they operate on an analogue scale. Therefore, this question targets your understanding of the binary nature of action potentials and their threshold, and what that means for the detection of below-threshold stimuli.  

Reference
- Messlinger, Anaesthesist (1997); 46(2): 142-53