Stomatal index 'I' is equal to?
A. $\dfrac{S}{{E - S}} \times 100$

B. $\dfrac{S}{{E + S}} \times 100$

C. $\dfrac{E}{{E + S}} \times 100$

D. $\dfrac{{E + S}}{E} \times 100$

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Hint: Stomata are the small pores. They are a crucial part of leaves and located on the underside of the leaves. These are accountable for transpiration and exchanges of gases in plants. The Stomatal index is known as the percentage in which the number of stomata forms to the total number of epidermal cells.
Stomatal index can be calculated by using the following equation.

$I = S \times \dfrac{{100}}{{E + S}}$
where I = Stomatal index
            S= Number of stomata per unit area
            E=Number of epidermal cells per unit area
Hence, the correct answer is option B, $\dfrac{S}{{E + S}} \times 100$

Complete answer:
Option (A) is not right because the Stomatal index is equal to no. Of stomatal cells divided by the total of stomatal and epithelial cells.

Option (B) is right. It states
In paleobotany, the ratio of the number of epidermal cells to the number of stomata in a given area of a leaf, times 100.Aug

Option (C) cannot be right. In the stomatal index, no. Of stomatal cells comes on the top of the division and not the no. Of epithelial cells.

Option (D) is wrong because the sum total of stomatal cells and epithelial cells comes at the bottom of the division.

Therefore, the answer is option B. $\dfrac{S}{{E + S}} \times 100$

Note: Stomata comprises a pair of specialized epidermal cells known as guard cells. Stomata regulates gas exchange between the plant and the environment. It also manages or controls the water loss by altering the size of the stomatal pore.