Answer:
The molar solubility of carbon dioxide gas is
.
Explanation:
Henry's law states that the amount of gas dissolved or molar solubility of gas is directly proportional to the partial pressure of the liquid.
To calculate the molar solubility, we use the equation given by Henry's law, which is:

where,
= Henry's constant = 
= partial pressure of carbonated drink

where = p = Total pressure = 0.400 atm
= mole fraction of 

Putting values in above equation, we get:

Hence, the molar solubility of carbon dioxide gas is
.
The third answer is the one you want. You have to have an adjustable density. All other things being equal, if the tanks you use for holding just water when filled with water will let the sub sink, because the sub is made of a dense metal like iron or steel.
If on the other hand you fill these tanks with air, the net density will be below one and the sub will rise.