To provide origin authentication, the sender encrypts the message using an asymmetric encryption algorithm. In this case, the receiver of the message must use what to decrypt the message?

A digital signature is the output artifact of the signing process, not the key material used to perform decryption.

A shared key implies symmetric cryptography, which does not apply to asymmetric origin authentication workflows.

The sender's private key is what was used to encrypt the message - using it again to decrypt would not work mathematically and would expose the sender's secret key.

Asymmetric key pairs are mathematically linked - data encrypted with one key can only be decrypted by the other. When a sender encrypts using their private key for origin authentication, the receiver uses the sender's public key to decrypt it, which proves the message came from the holder of that private key and no one else.