Question: What action does the MME take when it receives a BEARER RESOURCE MODIFICATION REQUEST message for an EPS bearer identity that does not belong to any already activated EPS bearer context?
Option 1: The MME sends a BEARER RESOURCE MODIFICATION ACCEPT message with ESM cause #0 "request accepted".
Option 2: The MME replies with a BEARER RESOURCE MODIFICATION REJECT message with ESM cause #43 "invalid EPS bearer identity".
Option 3: The MME discards the message and does not send any response.
Option 4: The MME sends a BEARER RESOURCE MODIFICATION REQUEST message to the Serving GW or PDN GW for further processing.
Explanation: According to the 3GPP specification, when the MME receives a BEARER RESOURCE MODIFICATION REQUEST message with an unknown EPS bearer context, it replies with a BEARER RESOURCE MODIFICATION REJECT message with ESM cause #43 "invalid EPS bearer identity". This is because the EPS bearer identity provided does not belong to any already activated EPS bearer context.