Final answer:
A recreational scuba tank should be filled with pure, filtered compressed air or Nitrox, not with pure oxygen or highly flammable gases such as hydrogen. Correct option is 3)
Step-by-step explanation:
A scuba tank for recreational diving should be filled mainly with pure, filtered compressed air or Nitrox (which is a mixture of nitrogen and oxygen with a higher oxygen content than air). Professional divers sometimes use a mixture called heliox, which is a combination of helium and oxygen, typically with a composition of 79% helium and 21% oxygen by mole. This mixture avoids the disoriented mental state known as nitrogen narcosis. It is not recommended to fill a recreational scuba tank with pure oxygen, as using pure oxygen at depth can lead to oxygen toxicity issues.
Regarding the other options given: Option 1 (a mixture of helium and oxygen) is used by professionals in specific conditions, option 2 (a mixture of hydrogen and oxygen) is not used due to hydrogen's flammability and potential for explosion, and option 4 (pure oxygen) is generally unsafe for recreational diving due to the risks of oxygen toxicity at the pressures experienced underwater.