What is the most likely cause of this error?

Refer to the exhibit.

A customer has an IPsec tunnel that is configured between two remote offices. The
customer is seeing these syslog messages on Router B:
%CRYPTO-4-PKT_REPLAY_ERR: decrypt: replay check failed connection id=x, sequence
number=y
What is the most likely cause of this error?

A.
The customer has an LLQ QoS policy that is configured on the WAN interface of
Router A.

B.
A hacker on the Internet is launching a spoofing attack.

C.
Router B has an incorrectly configured IP MTU value on the WAN interface.

D.
There is packet corruption in the network between Router A and Router B.

E.
Router A and Router B are not synchronized to the same timer source.