Explanation:

Let total capacity of the tank be \(60\) units.
If, \(\frac{1}{4}^{th}\) of the tank in already filled, we need to will rest of the tank i.e. \(\frac{3}{4}(60) = 45\) units

In \(1\) hour,
Work by A = \(\frac{60}{10} = 6\) units
Work by B = \(\frac{60}{20} = 3\) units
Work by C = \(\frac{60}{30} = -2\) units

NOTE: -ve sign indicates opposite work

\(7\) units of tank can be filled in \(3\) hours and at the same rate we can fill \(42\) units in \(6(3) = 18\) hours
Now, we need to fill \(3\) units using tap A

If tap A can fill \(6\) units of this tank in \(1\) hour then, it will take \(0.5\) hours to fill \(3\) units.

Col. A: 18 + 0.5 = 18.5 Hours
Col. B: 18 hours

Hence, option A