What is most likely to reduce the lumen of an intervertebral foramen?

Hypertrophy of the flaval ligament is the most likely factor to reduce the lumen of an intervertebral foramen because it involves the thickening of the ligament that sits within the vertebral canal. As this ligament enlarges, it encroaches upon the space within the foramina, which can subsequently compress the nerves passing through these openings. This situation can lead to neural compromise, contributing to conditions such as radiculopathy.

The other options can cause changes in the spinal structure, but they typically do not directly affect the intervertebral foramen lumen in the same manner. Compression of the nucleus pulposus relates more to disc bulging and does not have a direct effect on the foramen itself, while enlargement of the articular processes could potentially lead to osteophytic changes but doesn't as effectively narrow the foramen as ligamentous changes do. Calcification of the supraspinous ligament may contribute to overall spinal rigidity but does not directly narrow the intervertebral foramen space significantly.