Average of all even numbers between 104 and 148 is.

Answer: Option D

$$\begin{array}{rl}& \text{Average}=\frac{\text{First number}+\text{Last number}}{2}\\ & \text{So,}\\ & \text{First number}=104\\ & \text{Last number}=148\\ & \text{Average}=\frac{252}{2}=126\end{array}$$

Alternate:
Number of term between 104 and 148
146 = 106 + (n - 1)2
n = 21
Then, addition of all 21 term
$$\begin{array}{rl}& =\frac{21}{2}\left[2\times 106+\left(21-1\right)2\right]\\ & =\frac{21}{2}\left[212+40\right]\\ & =\frac{21}{2}\times 252\\ & \text{Average}=\frac{21}{2}\times \frac{252}{21}=126\end{array}$$