This is a 'range' question: we need to find out if the volume is more than 30 gallons. Unfortunately, there is usually no way around explicit calculations in these questions as the statement will also give us ranges and we'll need to see if they overlap or not. So we'll calculate -- a Precise approach.

(1) without information on the time, we cannot calculate anything.
Insufficient.

(2) without information on the rate of leakage, we cannot calculate anything.
Insufficient.

The maximal amount of water in the tank is 6.4 ounces/minute over 9.5 hours:
6.4 ounces/minute * 60 minutes/hours * 9.5 hours * 1/128 gallons/ounces = 6.4*60*9.5/128 gallons. 6.4*60 = 360 + 24 = 384, and 384*9.5 = about 3840 - 200 = 3640. Since 3640/128 < 30, the answer is definitively NO.

(C) it is.

Note that the types of calculations seen in statement (2) aren't very typical -- most GMAT calculation involve far friendlier numbers.
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