What is the most likely cause of this?

Routers R1 and R2 are configured for HSRP as shown below:

Router R1:
interface ethernet 0
ip address 20.6.2.1 255.255.255.0
standby 35 ip 20.6.2.21
standby 35 priority 100
interface ethernet 1
ip address 20.6.1.1.2 255.255.255.0
standby 34 ip 20.6.1.21

Router R2:
interface ethernet 0
ip address 20.6.2.2 255.255.255.0
standby 35 ip 20.6.2.21
interface ethernet 1
ip address 20.6.1.1.1 255.255.255.0
standby 34 ip 20.6.1.21
standby 34 priority 100

You have configured the routers R1 & R2 with HSRP. While debugging router R2 you notice very frequent HSRP group state transitions. What is the most likely cause of this?

A.
physical layer issues

B.
no spanning tree loops

C.
use of non-default HSRP timers

D.
failure to set the command standby 35 preempt

Explanation:
R2 is not able to from the standby state to reach the active state. This could be caused by missing HSRP hello messages. There are several possible causes for HSRP packets to get lost between the peers. The most common problems are Physical Layer Problems or excessive network traffic caused by Spanning-Tree Issues.
Note:
Hot Standby Routing Protocol (HSRP) is a Cisco proprietary protocol used for allowing redundant connections. It can keep core connectivity if the primary routing process fails. HSRP defines six states in which an HSRP router may run: initial, learn, listen, speak, standby, and active.
Incorrect Answers:
B: Spanning tree loops does not affect this problem.
C: Not a likely cause. Besides, in the example here the default values were indeed used.