A self-signed certificate is generated internally without involving an external Certificate Authority (CA). In a self-signed certificate, the certificate issuer and certificate subject are the same entity. Security+ SY0-701 explains that organizations frequently use self-signed certificates for internal systems, lab environments, or testing scenarios where external trust chains are unnecessary.
A digital signature (A) is a cryptographic function, not a certificate. Asymmetric keys (B) are used in public- key cryptography but do not constitute a certificate by themselves. Symmetric keys (D) are encryption tools, not certificates.
Therefore, the example of a certificate generated internally is C: Self-signed.