Asymmetric encryption is not suitable for

Asymmetric (public-key) cryptography uses a mathematically linked key pair — a public key and a private key — to deliver three classic security services: confidentiality (data encrypted with the recipient's public key can be decrypted only with the matching private key), authentication and non-repudiation (data signed with the sender's private key can be verified by anyone using the public key), and secure key exchange (safely agreeing on or transporting a shared secret over an untrusted channel, e.g. RSA key transport or Diffie–Hellman). Because it relies on expensive modular-exponentiation operations, it is far slower than symmetric encryption and is not built for line-rate, high-volume traffic handling.

Checking each listed use against this:

• Confidentiality — directly supported: encrypting with the recipient's public key restricts decryption to the holder of the matching private key.

• Authentication — directly supported: a digital signature created with the sender's private key lets any verifier confirm the sender's identity using the public key.

• Key Exchange — directly supported: protocols such as RSA key transport or Diffie–Hellman use asymmetric techniques specifically to establish a shared session key over an insecure network.

• Prevention of Denial of Service — not supported: defending against a flooding attack needs fast, stateless, line-rate controls (rate limiting, SYN cookies, traffic-pattern filtering), and asymmetric encryption's per-operation computational cost makes it impractical for this high-throughput task.

Cross-check: three of the four listed uses — confidentiality, authentication, and key exchange — are the textbook design goals of asymmetric cryptography, whereas stopping a flood of traffic is a network-layer, high-speed filtering problem, not a cryptographic one. This confirms only one use case is the odd one out.

So the use case asymmetric encryption is not suitable for is preventing Denial of Service attacks.