The series of consecutive numbers are x, (x+1), (x+2), (x+3).... and so on.

According to the formula,

The average of consecutive numbers= $\frac{\mathrm{First} \mathrm{number} +\mathrm{Last} \mathrm{number}}{2}$

Let First number= x

∴ 4th number (last number)=(x+3)

Given, average of 4 consecutive numbers = 20

Now, substitute these values in above formula,

$20=\frac{\mathrm{x}+\left(\mathrm{x}+3\right)}{2}$

⇒ 2x+3=40

⇒ 2x=37

∴ x= 18.5

 7th number = (x+6)

=(18.5+6)= 24.5

Hence, Option C is correct.