a. Tellurite, is a selective agent which is present in mitis-salivarius agar.
Mitis Salivarius Agar with Tellurite (MSAT) is a type of an enriched selective medium which has application in the isolation of Streptococcus salivarius, Streptococcus mitis and the other oral Enterococci and Streptococci which are found in different clinical specimens.
Peptones are a source of nitrogen, carbon, vitamins etc. and are added to the MSAT. Potassium tellurite and crystal violet are selective agents and can inhibit the growth of most gram negative bacilli and also the most gram positive bacteria excluding Streptococci.
Hence, option a. is the correct option.
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