What was the standard deviation if water at the end of the experiment?
-There are 6 water tanks
-Standard deviation at the beginning of the experiment = 10 gallons
Statement 1: For each tank, 30% of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment.
If each term or element in a set is increased or decreased by the same percent or constant value, the standard deviation will increase or decrease by the same percent or constant value.
So, if 30% of water is removed from each tank, the standard deviation will decrease by 30%.
Therefore, the standard deviation at the end of the experiment = 10 - (30% of 10) = $$10-\left(\frac{30}{100}\cdot10\right)=10-3=7$$
Statement 1 is, therefore, SUFFICIENT.

Statement 2: The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.
$$S\tan dard\ deviation=Summation\left(x_n-u\right)^2$$
This is how far the value of each element/tank is from the mean.
Since we do not know the exact volume of water in each tank, statement 2 is NOT SUFFICIENT.
Therefore, only statement 1 alone is SUFFICIENT, hence, the correct answer is option A.