B is correct because you don't need to enable parallel sharing rule calculation, it done by default.

https://developer.salesforce.com/docs/atlas.en-us.securityImplGuide.meta/securityImplGuide/security_sharing_recalculating_parallel.htm

When you update organization-wide defaults or sharing rules, automatic sharing recalculation is processed asynchronously and in parallel.

Parallel sharing rule calculation has the inbuilt capacity to start the updates from where it failed by having a control on queue mechanism. Correct option should be C

Answer is C:
https://developer.salesforce.com/docs/atlas.en-us.draes.meta/draes/draes_tools_parallel_sharing_rule_recalculation.htm

Parallel sharing will help the system recover from salesforce servers restart

Agree, B is about the ability to postpone a sharing recalculation while C is about making the recalculation faster

Ans C