A. Unstructured Supplementary Service Data
B. Unstable Supplementary Service Data
C. Under Supplementary Service Data
D. None of these

Solution

A. Unstructured Supplementary Service Data

Explanation

<p>USSD is a communication protocol used by GSM mobile phones to communicate with the mobile network operator’s computers.</p><p>It is commonly used for:</p><ul><li>Balance checks</li><li>Mobile banking</li><li>Recharge services</li><li>Accessing value-added services (VAS)</li></ul>