Mark works as an Office Assistant for uCertify Inc.

Mark works as an Office Assistant for uCertify Inc. He is responsible for managing office documents. Today, after opening a word document, Mark noticed that the other opened documents are closed suddenly. After reopening those documents, Mark found some modifications in the documents. He contacted his Security Administrator and came to know that there is a virus program installed in the operating system. Which of the following types of virus has attacked the operating system?