Which two protocols are used by bridges and/or switches to prevent loops in a layer 2 network? (Choose two.)

A.
802.1 d

B.
VTP

C.
802.1 q

D.
stp

E.
sap

Explanation:
This question is to examine the STP protocol.
STP (802.1 d) is used to prevent Layer 2 loops.
q is a Frame Relay protocol which belongs to VLAN. SAP is a concept of the OSI model.
So the correct answers are A and D.