Since the leak can empty the tank in 8 hours,
In 1 hour, part of the tank emptied $=\frac{1}{8}$

After opening the tap
In 1 hour, part of the tank emptied $=\frac{1}{12}$

Let us consider the tap fills the tank in x hours.
So, In 1 hour, part of the tank filled by the tap$=\frac{1}{x}$

According to the question,

$\Rightarrow \frac{1}{x}-\frac{1}{8}=\frac{1}{12}$

upon cross multiplication and solving we get x = 24

Since the tap admits 6 liters of water per hour, it will admit $6\times 24=144$liters of water

Hence option C is correct.