What is dissolved oxygen measurement used for?

Correct answer: To know available oxygen in the medium

Direct answer: Option A is correct. Dissolved oxygen measurement tells us how much oxygen is present in the liquid culture medium and available to the cells. This is especially important in aerobic fermentation, because cells consume oxygen while growing or producing a useful substance. The reasoning is: aeration adds oxygen; the liquid must receive and retain some of it; cells use it; measuring the dissolved amount helps the operator monitor whether oxygen is becoming too low and adjust air flow, stirring or other conditions. Option A exactly states this purpose. Option B, finding the smell of the medium, requires sensory or chemical analysis and is not what a dissolved-oxygen probe measures. Option C, finding fermenter height, is a physical measurement unrelated to oxygen concentration. Option D, choosing packaging colour, is a marketing or packaging decision and has no connection with dissolved oxygen. The reading is an indicator, not a complete assessment of growth; pH, temperature, nutrients and other factors also need control. Memory cue: DO means dissolved oxygen—oxygen dissolved in the liquid, not oxygen in the empty space above it.