Final answer:
The correct sentence is: Since they are adopting a baby, they have been shopping for strollers.
Step-by-step explanation:
The sentence with correct punctuation and the correct rule is:
Since they are adopting a baby, they have been shopping for strollers.
This sentence demonstrates the correct use of punctuation in a dependent clause. In this case, the dependent clause is 'Since they are adopting a baby', which is set apart from the main clause using a comma. This follows the rule of separating a dependent clause from the independent clause using a comma.
Learn more about Punctuation in English sentences