D. The coffee shop was unusual full, but today were many open tables.
A compound sentence is a sentence that has two complete sentences joined together using a comma and a conjunction. "The coffee shop was unusually full" is a complete sentence by itself. "Today [there] were many open tables" is also a complete sentence by itself. These are joined together with the conjunction "but" preceded by a comma.
The kids built a snow man that was better than mine
