Bacteria used to check the working efficiency of an autoclave is:: (A) Bacillus stearothermophilus, (B) Bacillus subtilis, (C) Campylobacter, (D) Bacillus tardinicus
Answer is A. There are three indicators that may be used to detect the efficacy of the autoclave process: 

Physical: Pressure and temperature recording devices.
Chemical: Indicators that change color after being exposed to specific temperatures, such as temperature-sensitive tape. The color change upon exposure to the given temperature.
Biological: Bacillus stearothermophilus spores are used, due to its resistance to heat, for the testing that measures the biological performance of the autoclave process.