Final answer:
Lock contention, caused by a)transactions competing for the same data rows, creates one of the most common bottlenecks in database transactions, leading to potential delays in processing.
Step-by-step explanation:
The question is regarding the common types of bottlenecks in database transactions. Among the options given, transactions competing for the same data rows creates one of the most common types of bottlenecks. This scenario, often referred to as 'lock contention,' occurs when multiple transactions are trying to access and modify the same data concurrently. The database management system must serialize access to the data rows to maintain data integrity, which can slow down or even block transaction processing until the locks are resolved.