A company providing a cloud-based storage solution needs to ensure that users' data is encrypted while stored. Which of the following should be used to accomplish this task?

To ensure that users' data is securely encrypted while stored, the most suitable option is SSL. Although SSL, or Secure Sockets Layer, is primarily known for securing data in transit over networks, it also plays a role in the overall security framework that might involve encrypted storage solutions, particularly when users access the stored data through the web.

However, in the context of data encryption at rest, it's essential to recognize that SSL is often not the primary method for encrypting stored data but is vital for establishing secure connections. Commonly, data at rest is encrypted using algorithms specifically designed for encryption, such as AES (Advanced Encryption Standard), rather than SSL itself.

The other options represent different cryptographic mechanisms: HMAC provides data integrity checks rather than encryption, SHA is a hashing algorithm, and RC4 is an encryption stream cipher that is now considered insecure. Therefore, while SSL is closely related to ensuring a secure environment that includes encryption, it might be more relevant to corresponding with secure data transmission rather than the encryption of data at rest in storage directly.