What is the name of the free program used for encrypting and decrypting emails?

The correct answer is PGP, which stands for Pretty Good Privacy. PGP is a widely-used encryption program that provides cryptographic privacy and authentication for data communication. It is primarily used for securing emails by encrypting the content so that only the intended recipient can read them. PGP uses a combination of symmetric and asymmetric cryptography to secure the emails, where a public key is used for encryption, and a private key is used for decryption. Additionally, it can also sign emails to verify the sender's identity.

The other options, while related to encryption in various contexts, do not serve the specific purpose of encrypting emails. SHA-1 and MD5 are cryptographic hash functions, primarily used for data integrity checks rather than for encrypting email content. DES (Data Encryption Standard) is a symmetric-key algorithm for encrypting digital data, but it predates PGP and is less commonly used on its own for email encryption in favor of more modern protocols and standards.