Penetrations in rated assemblies not properly firestopped can result in:

• A. No change to the assembly rating
• B. Improved energy efficiency
• C. Slight cosmetic improvement
• D. Loss of the assembly's fire resistance rating

Answer: (D)

The main idea here is that fire resistance relies on keeping flames, smoke, and heat contained within the designed barriers. When penetrations in a rated assembly aren’t properly firestopped, those openings become channels for heat and combustion products to pass through. That weakens the barrier during a fire and can cause it to fail to meet its tested duration, so the assembly loses its fire resistance rating. The other options don’t fit because improper firestopping doesn’t leave the rating unchanged, nor does it improve energy efficiency or provide any cosmetic benefit—the safety and performance of the assembly under fire are compromised.