What is a probable cause for this error?

Cloud Kicks, a global corporation, has the following environment:

– 50,000 users

– 24×7 call center operated by 25,000 users, including both employees and contractors

– A sales organization that has 15,000 users

The sales organization began processing about 120,000 updates to opportunity custom fields called priority and NextStep. They also started processing 20,000 updates to a highly nested territory hierarchy.

There was a third mass update on a Next Step field on the Action Plan custom object that has Case as a lookup field. Users started seeing a Group membership lock error in the system.

What is a probable cause for this error?
A . Due to system-initiated sharing rule recalculation, there is Lock contention
B . Due to Territory object updates, there is Lock contention on Territory object
C . Due to Action Plan custom object updates, there is Lock contention on Case records
D . Due to Opportunity object updates, there is Lock contention on Account records