Ans. is 'b' i.e., Bacillus stearothermophilus Steam sterilization is widely used as a terminal sterilization process for pharmaceutical drug products in glass ampoules, vials, syringes, and plastic containers.It is also used for sterilizing closures, filters, manufacturing equipment, cleaning equipment, and so forth. Biological indicatorsBiological indicators are "standardized" preparations of certain microorganisms with known characteristics (a defined population, purity and resistance).The microorganisms used to prepare biological indicators are those capable of forming endospores and the microorganism is used in the "spore state".The principal is that if these spores are destroyed it can be assumed that any contaminating microorganism in the sterilisation load would also have been killed.Different biological indicators are used for different sterilization processes. Biological indicators are designed for use with:Ethylene oxide gasHydrogen peroxide vapour (plasma)Dry heat sterilization (oven)Steam sterilization (autoclave)RadiationThe microorganism used will vary depending upon the means of sterilization which requires testing.Microorganisms are selected depending upon how resistant they are to the chosen method of sterilization.Different microorganisms are more resistant than others to different types of sterilization. With steam sterilization, for example, spore- bearing microorganisms are more resistant than non-spore bearing microorganisms.A microorganism like Staphylococcus (commonly carried on human skin) would have a typical D-value at 121oC for a 15- minute autoclave cycle of only 15 seconds, whereas an endospore forming thermophilic Bacillus would have one of at least 1.5 minutes.For steam sterilization, Geobacillus stearothermophilus (formerly described as Bacillus stearothermophilus) is most commonly used.This microorganism is used due to its theoretical resistance to particular types of sterilization, including heat and it is nonpathogenic.The most commonly used biological indicator in healthcare today contains endospores of Geobacillus stearothermophilus formerly called Bacillus stearothermophilus.The Geobacillus stearothermophilus bacterium is a nonpathogenic spore former that is able to with stand the high temperatures of steam sterilization when in its spore form.When compared to pathogenic species. G stearothermophilus survives much longer.This same organism is also used to monitor gas plasma sterilization, ozone sterilization and liquid sterilisation with peracetic acid because G. stearothermophilus is highly resistant to these sterilisation methods as well.The second organism used in healthcare facilities is Bacillus atrophaeus formerly called Bacillus subtilis var niger.This organism is resistant to ethylene oxide and dry heat sterilisation methods.The efficacy of the process has been established by demonstrating a sterility assurance level of at least 10'6 with Bacillus stearothermophilus.Both, the Bacillus stearothermophilus and B. subtilis provides high degree of resistance to any sterilisation process.Earlier there were discussions whether to use B. subtilis or B. stearothermophilus for plasma sterilization. This is no lower a topic of discussion because in February of 1999 a self-contained biological indicator using 10-6. Bacillus stearothermophilus became available internationally for the routine monitoring of plasma sterilization Steam cycles >Spores of geobacillus stearothermophilusEthylene oxide >Spores of bacillus atrophaeusPIasma sterlisation >Spores of geobacillus stearothermophilusDry heat >Spores of bacillus atrophaeusPeracetic acid >Spores of geobacillus stearothermophilusOzone cycles >Spores of geobacillus stearothermophilus