Answer: The correct expression is (A) 6b.
Step-by-step explanation: Given that Serena is writing an expression that is equivalent to (12ab - 6b), but she has not completed it.
The incomplete expression is ?(2a - 1).
We are to find the missing term from the above incomplete expression.
We have
Thus, the complete expression is 6b(2a - 1).
Option (A) is correct.