The CP75 is an audit notice regarding the tax return on which an Earned Income Credit was claimed. Here's a link to the IRS page which discusses the CP75: https://www.irs.gov/individuals/understanding-your-cp75-notice.
In the notice, IRS will give you specific instructions on how to provide the documentation they're seeking.
If you purchased Audit Defense, then you should contact the Audit Defense support agents prior to responding to IRS. Here's a link to the TurboTax audit support page, with contact information for Audit Defense: https://support.turbotax.intuit.com/irs-notice/audit-support/
