Which of the following are in descending order of their value ?

Answer: Option D

Converting each of the given fractions in to decimal form, we get
$$\begin{array}{rl}& \frac{1}{3}=0.33\\ & \frac{2}{5}=0.4\\ & \frac{3}{7}=0.42\\ & \frac{4}{5}=0.8\\ & \frac{5}{6}=0.83\\ & \frac{6}{7}=0.85\end{array}$$
Clearly, 0.85 > 0.83 > 0.8 > 0.42 > 0.4 > 0.33
So, $$\frac{6}{7}$$ > $$\frac{5}{6}$$ > $$\frac{4}{5}$$ > $$\frac{3}{7}$$ > $$\frac{2}{5}$$ > $$\frac{1}{3}$$