What does AP Echo Interval of 10 seconds indicate?

Refer to the exhibit.

What does AP Echo Interval of 10 seconds indicate?

A.
The access controller should send the access point (AP) an echo request every 10
seconds to ensure that the AP is still operational.

B.
The access point should send the access controller an echo request every 10 seconds to
ensure that the LWAPP tunnel is up.

C.
The access point should transmit a beacon every 10 seconds.

D.
The access point should send the client an echo request every 10 seconds to ensure that
the client is still connected.