Final answer:
The presence of blood or saliva in an impression can interfere with the casting material's setting process, leading to reduced surface detail and hardness of the cast.
Step-by-step explanation:
The presence of blood or saliva in an impression can indeed cause problems with the surface detail and surface hardness of a cast, so the correct answer is A. True. These bodily fluids can act as contaminants that affect the setting and curing of the materials used to create the cast. Blood or saliva can introduce moisture, which might interfere with the chemical reactions necessary for proper setting. This can result in a cast with inferior surface detail, making it less precise for the intended use, such as in dental prosthetics or forensic analysis.