Assertion. Data may be stolen through Eavesdropping.

Reason. Unauthorised monitoring of other people's communication is called Eavesdropping.

(a)

Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Explanation
When some one listens to a conversation that they are not part of, it is eavesdropping. Formally we can say that the intentional interception of someone else's data (such as email, login-id, password, credit card info etc.) as it passes through a user's computer to server or vice-versa is called eavesdropping. Data may be stolen through eavesdropping.