"their fulfillment" is correct and usable in written English.
You could use it when referring to the fulfillment of an individual or an organization's goals or objectives. For example: "The company focused its resources on achieving their fulfillment of the 10-year plan."
It simply demands their fulfillment.
Kafka's plans to move to Palestine grew more concrete only as their fulfillment grew less likely.
That it ended in the Terror represented a betrayal of those ideas not their fulfillment.
We are a little saddened because we begin to see the interval between our hopes and their fulfillment.
In addition, aggressive improvement targets have been established for each measure, and management bonuses are tied to their fulfillment.
Living under the covenant with Noah, their fulfillment of such responsibilities provides for their acceptance, for they are not expected to live within the realm of Torah.
A possible explanation for such results is that the WHL inscription generates positive expectations on both tourism demand and supply, their fulfillment depending on adequate local policies.
Concert composers keep their music going by a strategy of postponement, setting up expectations and delaying their fulfillment, but a film composer has to deliver the goods quickly.
The reality turned out to be much more ambiguous, as Mr. Khatami's tenure in office was marked more by the frustration of reformist aspirations than by their fulfillment.
Having gained consensus from her Cabinet on a realistic set of measurable goals, the president's next challenge was to influence their fulfillment.
These apply to all students in the Graduate School, although there are variations in the pattern of their fulfillment in individual departments and programs.
