Question 3
Why can water act as a respiratory medium for aquatic organisms?
The right answer is the one marked above.
Because oxygen is dissolved in the water. That dissolved oxygen is what aquatic organisms breathe.
“Because water contains oxygen atoms in its molecules” is a tempting trap. Water does contain oxygen atoms — the O in H2O — but those are locked into the water molecule and no animal can breathe them. What is breathed is the separate oxygen gas dissolved in the water.