This question is about identifying the country behind the COMCASA agreement with India. Sri Lanka, Japan and Russia all maintain defence relationships with India in some form, but COMCASA specifically is not an agreement India has with any of these three, so they can be dismissed together.
COMCASA, short for Communications Compatibility and Security Agreement, is a pact between India and the United States that let the two navies set up their first secure, real time communication link, improving coordination during joint exercises and operations.
So the correct answer is US.
Communications Compatibility and Security Agreement, or COMCASA, is one of the three foundational defense pacts India signed with a key partner nation to enable the exchange of encrypted and secure military communications equipment. Knowing what the agreement covers helps identify which country it belongs to.
Reviewing the actual scope of the COMCASA agreement makes it clear that the US is the country involved.
Therefore, the correct answer is US.