Explanation:
The tool that the penetration tester should use for further investigation is WPScan. This is because WPScan is a WordPress vulnerability scanner that can detect common WordPress security issues, such as weak passwords, outdated plugins, and misconfigured settings. WPScan can also enumerate WordPress users, themes, and plugins from the robots.txt file.
The two entries in the robots.txt file that the penetration tester should recommend for removal are:
Allow: /admin
Allow: /wp-admin
These entries expose the WordPress admin panel, which can be a target for brute-force attacks, SQL injection, and other exploits. Removing these entries can help prevent unauthorized access to the web application's backend. Alternatively, the penetration tester can suggest renaming the admin panel to a less obvious name, or adding authentication methods such as two-factor authentication or IP whitelisting.