The Request to Send (RTS) and Clear to Send (CTS) frames are part of the 802.11 Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism, specifically the Distributed Coordination Function (DCF). When a station (STA) intends to send a data frame that exceeds the configured RTS Threshold, it first transmits an RTS frame to the intended recipient. The recipient STA, upon receiving the RTS, responds with a CTS frame after a Short Interframe Space (SIFS). This CTS frame serves as the positive acknowledgment to the RTS and reserves the radio frequency (RF) medium for the subsequent data frame transmission, helping to mitigate the hidden node problem.

B. Ack: An Ack frame is used to acknowledge the successful reception of unicast data or management frames, not control frames like RTS.

C. RTS-Ack: This is not a defined frame type within the IEEE 802.11 standard; it is a fabricated option.

D. Block Ack: A Block Ack is a mechanism introduced in 802.11n to acknowledge a group of frames (an A-MPDU) at once, not a single RTS frame.

1. IEEE Computer Society. (2021). IEEE Standard for Information Technology--Telecommunications and Information Exchange between Systems Local and Metropolitan Area Networks--Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. IEEE Std 802.11™-2020.

Section 10.3.2.7, "RTS/CTS procedure," states: "A STA may transmit an RTS frame when it desires to transmit a frame. Upon receiving an RTS frame, the destination STA shall respond with a CTS frame after a SIFS period if the NAV at the destination STA is clear." This directly specifies that a CTS is the response to an RTS.

2. Carnegie Mellon University. (2005). 18-759 Wireless Networks, Lecture 4: MAC Layer I: 802.11. School of Electrical and Computer Engineering.

Slide 23, "RTS/CTS Handshake," illustrates the exchange sequence: "Sender sends RTS (Request to Send). Receiver responds with CTS (Clear to Send)." This confirms the fundamental protocol interaction where CTS acknowledges the RTS.