A Transitional Security Network (TSN) allows legacy stations to interoperate by using older encryption methods. If AES (CCMP) is unsupported by older equipment, the network can fall back to TKIP, which uses RC4 as its encryption algorithm. TKIP enables AES encryption on newer devices while accommodating legacy clients.
Options A, C, D are current or deprecated standards with AES; only RC4 matches the transitional need.
References:
CWSP#207 Study Guide, Chapter 3 (TSN, TKIP, AES-CCMP)