Bitcoin, Litecoin and Monero.
Mars accepts three coins, each for a reason.
Monero is the privacy default. Litecoin is the small-order coin, faster and cheaper than Bitcoin when the network fee would be out of proportion. Bitcoin is there for the vendors who still price in it.
Default to Monero unless a vendor forces otherwise. If you pay in Bitcoin, route through a fresh wallet rather than sending straight from an identity-checked exchange. Deposit addresses are per order.
Mars Market