Ultimately, you must choose the SAQ that’s right for your processing environment, but generally speaking:
SAQ A is for e-commerce/mail/telephone-order (card-not-present) merchants that have fully outsourced all cardholder data functions. No electronic storage, processing or transmission of any cardholder data on the merchant’s systems or premises.
SAQ A-EP is for e-commerce-only merchants that use a third-party service provider to handle their card information, and who have a website that doesn’t handle card data, but could impact the security of the payment transaction. No electronic storage, processing or transmission of any cardholder data on the merchant’s systems or premises.
SAQ B is for merchants that use imprint machines and/or standalone, dial-out terminals, and have no electronic cardholder data storage. Not for e-commerce.
SAQ B-IP is for merchants using only standalone, PTS-approved payment terminals with an IP connection to the payment processor, and that have no electronic cardholder data storage. It's not for e-commerce.
SAQ C-VT is for merchants that use a virtual terminal on one computer dedicated solely to card processing. There is no electronic cardholder data storage. It's not for e-commerce.
SAQ C is for any merchant with a payment application connected to the Internet, but there is no electronic cardholder data storage.
SAQ D for Merchants is for merchants that DO store credit card data electronically.