Neutral water with pH about 7 becomes slightly acidic when aerated. This is because:

• A

  oxygen from air is dissolved in the water which makes the water acidic
• B

  dirt, which get contaminated with the water during aeration makes the water acidic
• C

  ultraviolet radiation dissociates water molecules and makes water acidic
• Correct answer
  D

  carbon-dioxide from air is dissolved

Hence, the correct answer is option: 'D'.