After a technician installs a new motherboard, the computer will not start and fails POST.

Comprehensive and Detailed In-Depth Explanation:

A common reason for a POST failure after a motherboard installation is improperly seated RAM. The system requires working memory to successfully complete POST. Reseating (removing and reinserting) the RAM ensures it's making proper contact with the motherboard.

Option A: Flashing the BIOS is not possible if the system won’t POST.

Option B: A dead CMOS battery can cause BIOS settings to reset, but it usually won’t prevent POST entirely.

Option D: Reinstalling the old motherboard is premature — basic troubleshooting steps should be completed first.

CompTIA A+ Core 1 Exam Objective Reference:

Objective 3.5: Given a scenario, troubleshoot problems related to motherboards, RAM, CPU, and power.