Final answer:
Sugar was added to a petri dish as a nutrient to observe the effect on the growth and metabolism of microorganisms.
Step-by-step explanation:
The sugar put in the petri dish in the given procedure was most likely used as a nutrient to test the effect of sugar on microorganisms, which is option B. In a biology laboratory setting, growth media like agar, which provide essential nutrients for microorganisms, may have additives like sugar to grow specific types of organisms or to observe their metabolic activities. Sugar, being a source of carbon and energy, would allow the microorganisms to grow, therefore serving as an important component in the observation of the bacteria's behavior or growth characteristics.