You've basically got it now. The answer to your original question was D. Other - programmed VNAV speed.
The programmed speed is displayed on VNAV DESCENT page, as well as LEGS page. Typically it is the 240/10000 restraint, followed by any restraint programmed on the legs page or from the STAR/Approach. A simple approach might be the 240/10000 restriction, 220/3000 at the IAF and 180/2000 FAF. If you don't speed intervene, VNAV will fly you at 240K until approaching the IAF, then slow to 220K and slow again approaching the FAF to achieve the 180K constraint.