There are two facts given:
1. Urban air has more sulphur dioxide.
2. Urban plants grow slower.

We want to find the effect of high sulphur dioxide in the air on the slow growth. Say, urban plants grow at the rate of 2 cm per month. Rural grow at 8 cm per month.
If we remove sulphur dioxide from air, we might see that urban plants grow at 6 cm per month while rural grow at 10 cm per month. This will show us the effect of higher sulphur dioxide. But these numbers will be dependable when everything else is kept the same. Only then we can separate out the contribution of higher levels of sulphur dioxide. If the urban greenhouse is different from the rural greenhouse, the difference could account for a part of the difference in growth rate too. Option (D) tells us that urban greenhouse gets lesser light. This means we cannot measure the contribution of higher sulphur dioxide to slower growth rate. Hence we need option (D) to evaluate the result.