Correct answer is:

a) 50 days

Explanation

Here,
Hours are inversely proportional to Days.
And,
Length, Breadth and Depth are Directly proportional to Days.
\therefore x=10\times\large\frac{12}{8}\normalsize\times\large\frac{140}{70}\normalsize\times\large\frac{4}{3}\normalsize\times\large\frac{2.5}{2}
\rightarrow x=10\times\large\frac{12}{8}\normalsize\times\large\frac{140}{70}\normalsize\times\large\frac{4}{3}\normalsize\times\large\frac{25}{2\times10}
\rightarrow x=50 days.
Ans: If the workers dig a canal with a length of 140 meters, breadth of 4 meters, and depth of 2.5 meters while working 8 hours per day, it will take them 50 days to complete the task.