Final answer:
A contaminant in a liquid culture is generally more problematic than on a streak plate because it can quickly affect the entire culture, making isolation of a pure culture more difficult.
Step-by-step explanation:
The effect of a contaminant landing on a streak plate versus in a liquid culture varies significantly. On a streak plate, the contaminant may lead to the growth of unwanted colonies, but due to the streaking technique, which aims to isolate individual colonies, it might be possible to still obtain a pure culture by selecting uncontaminated sectors. However, in a liquid culture, a contaminant can rapidly disperse throughout the medium, potentially affecting all the cells and making it much more challenging to isolate a pure culture. Therefore, a contaminant in a liquid culture generally poses more of a problem. The answer to the student's question would be b. Liquid culture.