In Juniper Networks Mist AI Wireless, the behavior described in this scenario is explained by dynamic rate shifting. Dynamic rate shifting is the mechanism by which a wireless client and access point automatically adjust the modulation and coding scheme (MCS) based on current RF conditions to maintain a reliable connection.

When a client is close to an access point, RF conditions are optimal---high RSSI, high signal-to-noise ratio (SNR), and low error rates---allowing the client to use higher-order modulation schemes such as 1024-QAM (in 802.11ax). These modulation schemes deliver higher throughput but require very clean RF conditions. As the client moves farther away from the AP, signal strength decreases and noise becomes more impactful, increasing frame errors and retransmissions.

Dynamic rate shifting continuously evaluates transmission success, retry rates, and acknowledgments. When errors increase, the client steps down to lower modulation rates (for example, from 1024-QAM to 256-QAM or 64-QAM) that are more resilient to reduced signal quality. This trade-off sacrifices peak throughput in favor of connection stability and reliability.

The other options describe contributing factors, but not the mechanism itself:

RSSI indicates signal strength but does not control rate selection.

SNR influences modulation decisions but is not the adaptive process.

Free space path loss explains why signal weakens over distance but does not describe how rates are adjusted.

Mist AI Wireless monitors modulation rates, retries, and throughput via advanced telemetry and uses this data in Wireless Assurance SLEs to highlight RF and performance issues. However, the actual process of stepping up or down transmission rates in response to changing RF conditions is governed by dynamic rate shifting.

Therefore, the correct answer is C. Dynamic rate shifting.