Select the option that is related to the third term in the same way as the second term is related to the first term and sixth term is related to fifth term.

27 : 6 :: 8 : ? :: 64 : 8

The logic here is

$$\left(\frac{6}{2}\right)^3=3^3=27$$ $$\longrightarrow\ $$ $$\left(\frac{\text{second number}}{2}\right)^3=\text{first number}$$

$$\left(\frac{8}{2}\right)^3=4^3=64$$ $$\longrightarrow\ $$ $$\left(\frac{\text{sixth number}}{2}\right)^3=\text{fifth number}$$

Let the fourth number = $$a$$

Similarly, $$\left(\frac{a}{2}\right)^3=8$$

$$\Rightarrow$$ $$\frac{a^3}{8}=8$$

$$\Rightarrow$$ $$a^3=64$$

$$\Rightarrow$$ $$a=4$$

$$\therefore\ $$The fourth number = 4

Hence, the correct answer is Option D