Encryption used by RADIUS is BEST described as:

A.
Quantum

B.
Elliptical curve

C.
Asymmetric

D.
Symmetric

Explanation:
The RADIUS server uses a symmetric encryption method.
Note: Symmetric algorithms require both ends of an encrypted message to have the same key and processing algorithms. Symmetric algorithms generate a secret
key that must be protected.
Incorrect Answers:
A: Quantum encryption is a hypothetical encryption method not yet in use.
B: Elliptical curve cryptography (ECC) is an approach to public-key cryptography, but the RADIUS protocol uses a private(secret) key.
C: RADIUS uses a symmetric, not an asymmetric, encryption method.

Dulaney, Emmett and Chuck Eastton, CompTIA Security+ Study Guide, Sixth Edition, Sybex, Indianapolis, 2014, pp 249-251, 251-254, 257
http://www.studymode.com/essays/Elliptic-Curve-Cryptography-And-Its-Applications- 1560318.html
http://en.wikipedia.org/wiki/Elliptic_curve_cryptography