What transport layer protocol does ping use?

Assuming IPv4 ICMP. Ping uses IP for transport. Ping in itself is an "Echo Request", which is a function of the ICMP protocol. The IP Packet will carry the ICMP protocol from end-to-end.

ICMP has a protocol number of 1, so the "Protocol" field in the IPv4 header will contain "1" as a reference to the ICMP payload.

For IPv6, ICMPv6 is used, and the transport will be IPv6.