A user ' s monitor shows strange colored lines and flickers when adjusting the brightness.

The correct answer is D. Check for loose connections on the display.

According to CompTIA A+ Core 1 220-1201 objectives under Hardware and Display Troubleshooting:

Flickering or strange colored lines on a monitor are often caused by loose or damaged video cable connections (internal or external).

Before considering costly or time-consuming solutions like replacement, the technician should verify that the display cable, internal ribbon cables, or connectors are properly seated.

Why the other options are incorrect:

A. Suggest a secondary monitor: Provides a temporary workaround but does not resolve the underlying hardware issue.

B. Conduct a factory reset: Resets software settings but will not correct physical connection problems that cause flickering or colored lines.

C. Install a screen protector: Does not affect video signal or display functionality; irrelevant for flickering issues.