Bacillus used to test the efficacy of Sterilization by Autoclave is?: (A) Bacillus subtilis., (B) Bacillus pumilus, (C) Bacillus stearothermophilus, (D) Coxiella burnetii
Answer is C. Various bacilli are used to check the efficacy of sterilization methods are:

Autoclave- Bacillus stearothermophilus
Hot air oven- Bacillus subtilis subspecies niger
Ionising radiation- Bacillus pumilus