Which of the following are true for SdfChangeBlocks?

Strictly evaluated against the OpenUSD API contract, C is the only fully correct statement as written. SdfChangeBlock groups multiple low-level Sdf edits so that change processing can be delayed and handled efficiently as one batch. The official API documentation states that opening a change block causes Sdf to delay notifications until the outermost block exits; notifications are queued, not permanently muted. Therefore, A is incorrect. The same documentation gives a strong warning that it is not safe to use Usd or other downstream APIs while a change block is open, because those representations may be stale or may hold expired handles to Sdf objects. Therefore, querying mutating prims during the block is unsafe, making C correct.

Option B is incorrect because it explicitly says to use USD APIs inside the block, which the official guidance warns against. Option D is also incorrect because the intended safe authoring pattern is to gather required edits first, then apply them directly through the Sdf API inside the change block. This aligns with Pipeline Development → Efficient Authoring, Sdf Layers, SdfChangeBlock, Change Notification, and Bulk Scene Description Mutation .