The correct answer is B. Change the resolution .

According to the CompTIA A+ Core 1 220-1201 objectives, troubleshooting display issues involves checking the display settings, including resolution, scaling, and orientation. When a projector is only showing part of a laptop screen:

The most common cause is that the laptop’s output resolution does not match the projector ' s native resolution. If the laptop is set to a resolution larger than what the projector supports, only a portion of the screen is visible.

The technician should first adjust the laptop ' s resolution to match the projector’s native resolution before attempting other options like extending or mirroring the desktop.

Why the other options are incorrect:

A. Extend the desktop : This is only used if the user wants to have two separate screens (dual monitors), not to fix partial display issues.

C. Mirror the desktop : Mirroring duplicates the laptop display but does not solve issues caused by resolution mismatch.

D. Switch the input source : Necessary only if the projector is on the wrong input, but the problem here is a partial display, not no display.