c. Bulk API

Inserting or updating records through the Bulk API can cause multiple updates on the same parent record at once, because the batches are processed in parallel. For example, if two batches are being processed at the same time and the two of them contain records pointing to the same parent record, one of the batches will attempt to place a lock in the parent record, which can lead to the other batch throwing a "unable to lock row" error as the batch was not able to get a lock within 10 seconds.

To prevent this, you can do either of the following:
Reduce the batch size
Process the records in Serial mode instead of parallel, that way one batch is processed at a time.
Sort main records based on their parent record, to avoid having different child records (with the same parent) in different batches when using parallel mode.

https://help.salesforce.com/s/articleView?id=000387767&type=1

Answer should be C according to the Salesforce Help documentation

C is True