The correct answer is Encrypting objects at rest with strong cryptography.

Encrypting objects at rest with strong cryptography protects the confidentiality and integrity of stored data but it does not reduce request volume or network traffic that cause a denial of service. Encryption ensures that data cannot be read or tampered with by unauthorized parties, but it does not stop floods of requests, bandwidth saturation, or resource exhaustion that lead to service interruptions.

Using a content delivery network to cache frequently accessed objects is incorrect because a CDN reduces load on origin storage by serving cached copies at edge locations. A CDN absorbs and distributes traffic and it is commonly used to mitigate volumetric and application layer denial of service attacks.

Implementing request throttling and bandwidth limits is incorrect because rate limiting and throttling directly limit how many requests a client can make and how much bandwidth a user can consume. These controls protect backend storage from being overwhelmed and are effective for reducing the impact of abusive or accidental traffic spikes.

Automatically scaling storage and compute resources to absorb spikes in traffic is incorrect because elasticity increases capacity to handle sudden increases in demand. Auto scaling can reduce the chance of outages during legitimate traffic spikes and it can help absorb some forms of denial of service attacks, although it may not stop all large scale volumetric attacks or network saturation.

Full AWS Practitioner Certification Question