Refer to the exhbit.

 The Time to Live (TTL) security mechanism is a lightweight security feature that can protect against CPU and memory resource exhaustion due to a denial-of-service (DoS) attack. It works by setting a limit on the number of hops (TTL value) that BGP packets can traverse. By configuring neighbor 10.163.83.55 ttl-security hops 2, ISP A ensures that BGP packets received from ISP C must have a TTL value of at least 254 when they reach R1, as BGP packets decrement the TTL by 1 with each hop. This effectively prevents spoofing attacks from outside the directly connected network because any packets spoofed from further away would have a TTL that drops below the threshold before reaching R1.

References: This explanation is based on the security features described in the Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR)