Patient has a ganglion cyst, which can spontaneously regress.

FA 2019 page 453: Ganglion cyst = Fluid-filled swelling overlying joint or tendon sheath, most commonly at the dorsal side of the wrist. Arises from herniation of dense connective tissue..

FA doesn't discuss long-term prognosis/course.

Per this article (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2682407/), the ganglion cysts fluid is due to extra-articular mucin droplet accumulation. It also indicates 50% undergo spontaneous resolution. On examination, wrist ganglion are usually 1–2 cm cystic structures, feeling much like a firm rubber ball that is well tethered in place by its attachment to the underlying joint capsule or tendon sheath. There is no associated warmth or erythema and the cyst readily transilluminates.

To rule out the other options:

A. Per the above article, these aren't really cysts because there is no lining: "It should be emphasized that since no synovial lining exists in these structures, they cannot be classified as true cysts. Though there are focal areas of mucinous degeneration in the cyst wall, neither significant global degenerative changes, necrosis nor inflammatory changes within the pseudocyst or surrounding tissues have been demonstrated" = A is wrong as it wouldn't have malignant potential.

B. Honestly not sure how to rule this answer out because if it compressed the superficial radial nerve branches, theoretically it could cause paresthesias. That being said, the couple of articles I've read have only indicated potential paresthesia due to ganglion cyst being due to dorsal cysts, NOT volar cysts. C. Based on these pictures (http://bit.ly/30hccjo, http://bit.ly/30loCH4, http://bit.ly/30kFrBV, and http://bit.ly/30qahZN), volar ganglion cysts (as this patient has) do not cause extensor retinaculum nerve entrapment, and thus are unlikely to produce radial nerve palsy. Also, the radial nerves closes to a volar ganglion cyst, seem to be cutaneous as opposed to motor, so if they did cause compression, it would likely only lead to sensory deficit, not motor deficit.

D. Yes, 50% spontaneously regress.

E. Synovitis = no because it is not an inflammatory condition