Which measure is correct to prevent contamination?

Correct answer: Sterilization of equipment and medium

The direct answer is A: sterilization of the equipment and culture medium. Sterilization means treating materials so that unwanted microorganisms are destroyed or removed before the process begins. In a bioprocess, the desired cells or organism must grow without competition. First, equipment is cleaned; then suitable heat, steam, filtration, or another validated method is used. The medium is also sterilized, and sterile handling is maintained afterward. Thus, equipment plus medium sterilization breaks the contamination chain. Option A is correct because it protects the culture from unwanted bacteria, fungi, and other organisms. Option B is wrong because leaving a fermenter open allows microbes and dust to enter. Option C is wrong because dirty water introduces microorganisms and impurities. Option D is wrong because adding an unwanted culture deliberately introduces a contaminant and may spoil the batch. The useful exam cue is: sterile equipment plus sterile medium means a safer pure culture.