Answer Description

In PKI data encrypted using a public key can only be decrypted by the public key's private pair. James needs to send the email so only Bob can decrypt it, so he will use Bob's public key. When Bob gets the email he will use his private key to decrypt it, therefor ensuring confidentiality.

Wikipedia

Email encryption is encryption of email messages to protect the content from being read by entities other than the intended recipients. Email encryption may also include authentication. Email is prone to the disclosure of information. Most emails are encrypted during transmission, but they are stored in clear text, making them readable by third parties such as email providers. By default, popular email services such as Gmail and Outlook do not enable end-to-end encryption. By means of some available tools, persons other than the designated recipients can read the email contents.Email encryption can rely on public-key cryptography, in which users can each publish a public key that others can use to encrypt messages to them, while keeping secret a private key they can use to decrypt such messages or to digitally encrypt and sign messages they send.