Which one of the following options best describes the main purpose of a Stage 2...

According to ISO/IEC 17021-1:2015 (used alongside ISO 14001 for certification audits), the certification process includes Stage 1 and Stage 2 audits:

The purpose of the Stage 1 audit is to review the management system documentation and assess the organization's preparedness for Stage 2.

The Stage 2 audit’s main objective is to evaluate the implementation, including the effectiveness, of the organization’s management system. This includes assessing conformity with all the requirements of ISO 14001:2015.

As per ISO/IEC 17021-1:2015 Clause 9.3.2.2.2:

"The stage 2 audit shall take place at the site(s) of the organization. It shall include at least the following: information and evidence about conformity to all requirements of the applicable management system standard."

Thus, the Stage 2 audit is where auditors identify nonconformities and gather evidence to support certification decisions.