You are conducting an experiment to determine whether lichen populations are affected by local airborne pollutants. Which of the following observations would best support your hypothesis that high levels of airborne pollutants cause a reduction in lichen abundance:

a) lichens growing in areas with high levels of SO2 tend to grow in smaller, but more numerous colonies
b) areas with high NO2 levels tend to have fewer of the species of tree that lichens prefer to grow on
c) lichens are abundant where rainfall has been determined to be low in dissolved pollutants
d) lichens in suspected polluted areas have a significant increase in the proportion of sulphur and nitrogen absorbed
e) none of the above observations would support my hypothesis

The correct answer is d) lichens in suspected polluted areas have a significant increase in the proportion of sulphur and nitrogen absorbed.

To determine whether high levels of airborne pollutants cause a reduction in lichen abundance, you need to look for evidence that lichens in polluted areas are affected by the pollutants. This can be done by studying the absorption of pollutants by lichens.

To conduct this experiment, you would need to collect lichen samples from areas suspected to be polluted and compare the proportion of absorbed sulphur and nitrogen in these samples with those collected from areas with lower pollution levels.

To analyze the samples, you would need to measure the concentration of sulphur and nitrogen in the lichen tissues and compare the results between the two groups. If the lichens in the suspected polluted areas have a significantly higher proportion of sulphur and nitrogen absorbed, this would support the hypothesis that high levels of airborne pollutants cause a reduction in lichen abundance.

Observations a), b), and c) do provide some information related to pollution and lichens, but they do not directly measure the absorption of pollutants by lichens. Therefore, they may not be as strong of evidence for the hypothesis as observation d), which directly measures the increase in absorbed pollutants in lichens from suspected polluted areas. Therefore, the correct answer is d) lichens in suspected polluted areas have a significant increase in the proportion of sulphur and nitrogen absorbed.