Final answer:
The correct sentence with punctuation is 'Send a card to 1234 Broad St., Washington, D.C., to let us know what you think.' The commas are used correctly to separate the address and location.
Step-by-step explanation:
The correct sentence with punctuation is:
Send a card to 1234 Broad St., Washington, D.C., to let us know what you think.
In this sentence, the commas are correctly used to separate the different elements of the address (1234 Broad St.) and the location (Washington, D.C.). The punctuation is also correct after the second comma to separate the address from the purpose of sending the card ('to let us know what you think').