Here we can use the formula

$$\begin{gathered} \text{Average (A)=}\frac{\mathrm{Sum} \mathrm{of} \mathrm{observations} \left(\mathrm{S}\right)}{\mathrm{No}. \mathrm{of} \mathrm{observations} \left(\mathrm{N}\right)} \\ \text{⇒ S=A×N} \end{gathered}$$

According to the question,

Sum of height of 100 big trees and a banyan tree (A=3, N=101)

⇒S=3 X 101 =303

Later, Height of the banyan tree is not consider, the average decreases by 2 meters. (so A=1, N=100)

So, sum of 100 big trees

⇒S=1 X 100 =100

Now,

The height of banyan tree =(sum of 7 observations)-(Sum of 6 observations)

=303-100

=203

Hence, Option A is correct.