Answer:
The correct sentence is: "Wow, that is a great idea! I'd love to join you but I have a softball tournament that day."
Step-by-step explanation:
A sentence always starts with a capital letter. Therefore, the "W" at the start is capitalised. A comma is put after the first word because "Wow" is an exclamation word, after which the actual statement is said.
"I" is also always capitalised when used in a sentence. At the end, a full stop is added and the quotation marks are closed.