**Core Concept**
The ADA specification number 5 outlines the requirements for the composition of gold alloys used in dental restorations, including the minimum percentage of precious metal content. Precious metals in dental gold alloys serve as the primary component, imparting strength, durability, and corrosion resistance to the cast restorations.

**Why the Correct Answer is Right**
The correct answer, 75%, reflects the minimum percentage of precious metal required by the ADA specification number 5. This ensures that the gold alloy has sufficient precious metal content to provide the necessary mechanical properties, such as strength and corrosion resistance, for dental restorations. The presence of precious metals like gold, silver, and palladium contributes to the alloy's biocompatibility and resistance to corrosion, making it suitable for use in the oral environment.

**Why Each Wrong Option is Incorrect**
**Option A:** 55% is too low, as it would compromise the mechanical properties of the gold alloy, potentially leading to inadequate strength and durability in dental restorations.
**Option B:** 65% is still below the minimum required by the ADA specification number 5, which may result in suboptimal performance of the cast restoration.
**Option D:** 85% exceeds the minimum percentage required by the ADA specification number 5, but specifying a higher percentage is not necessary for achieving the required mechanical properties.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the ADA specification number 5 is a widely accepted standard for dental gold alloys, and compliance with this specification ensures that the alloy meets the necessary requirements for strength, durability, and corrosion resistance in dental restorations.

**✓ Correct Answer: C. 75**