Kickstart Securities provides network analysis and safety measurement services to various clients. Recently, one of its clients has requested a technician to run a cross-check on the network system to ensure that there are no vulnerabilities that are exposed. You have been sent in to assess the situation and fix the errors if there are any. Which one of the following will you use in such a scenario to track the vulnerabilities across systems?

a.Quid pro quo

b.Phishing

c.Zero-day exploit

d.CVE (Common Vulnerabilities and Exposures)