To determine the height in microns of the plant cells after 2.5 seconds, we need additional information or data about the growth pattern or measurement taken by the scientist. Without the specific growth data or a formula that relates time to height, it's not possible to accurately answer the question.
However, if those responses (10, 12, 21, 29) are options given based on some data you've encountered, I would suggest selecting the most likely or applying any known growth rules or patterns observed in similar situations.
Could you provide any additional details or data, such as a growth equation or previous measurements?