Which sentences use commas correctly?

The sentence that uses commas correctly is the one stating that customers are complaining, so we need to take action. This sentence employs a comma before the coordinating conjunction "so," which correctly separates the two independent clauses: "Customers are complaining" and "we need to take action." Using a comma in this context helps clarify the relationship between the two thoughts by indicating a pause, making the overall meaning clearer.

In contrast, other options fail to use commas appropriately. The second option lacks commas necessary to set off the restrictive clause, which should be punctuated as "The meeting, which started at 10, was very productive," indicating that the additional information is non-restrictive. The third option should also include a comma before "but," which would clarify the transition from an assertion to a contrasting idea. Finally, the fourth option misuses punctuation by not providing the necessary comma before "who's," which should be written as "Ted, who's very experienced, will be the project director," to correctly separate the clause that describes Ted.