Which pair represents two forms of an element in the same phase at stp but with different structures and different properties? (1) i2(s) and i2(g) (3) h2(g) and hg (g)
(2) o2(g) and o3(g) (4) h2o(s) and h2o(l)

In this question, you are asked to find two condition which was:
1. Same phase at STP( standard temperature and pressure)
2. Different structure and properties

In option 1 and 4 the phase of the element is different. It was solid+gas for option 1 and solid+liquid for option 4. That clearly not fulfills the first condition. 
At STP helium is gas but mercury should be liquid, not gas. That means option 3 won't fulfill the 1st condition too.

Option 2 is true since both of them gas and their structure and properties are different. One example of properties is that oxygen doesn't have an odor but ozone has.

Ithink 2 is the correct