John, a disgruntled ex-employee of an organization, contacted a professional hacker to exploit the organization. In the attack process, the professional hacker installed a scanner on a machine belonging to one of the victims and scanned several machines on the same network to identify vulnerabilities to perform further exploitation.
What is the type of vulnerability assessment tool employed by John in the above scenario?

A. Agent-based scanner
B. Network-based scanner
C. Cluster scanner
D. Proxy scanner