Which element is most often used as the foundation for a hardware root of trust for a modern PC?

Modern PCs rely on a Trusted Platform Module (TPM) as the basis for a hardware root of trust. The TPM may furnish a cryptographic key, a unique serial number for the device, or a physically unclonable function (PUF). Unlike the CPU and hard drive, the TPM is responsible for this function. While Hardware Security Modules (HSMs) are utilized for PKI and cryptographic purposes, they are not used as a hardware root of trust for PCs.