Which reaction will occur?
2nabr + i2 mc023-1.jpg 2nai + br2
2fe + al2o3 mc023-2.jpg 2al + fe2o3
2agno3 + ni mc023-3.jpg ni(no3)2 + 2ag
pb + zn(c2h3o2)2 mc023-4.jpg zn + pb(c2h3o2)2

The reactions that will occur are and

Explanation:

Single displacement reaction is defined as the reaction in which more reactive element displaces a less reactive element from its chemical reaction.

The reactivity of metal is determined by a series known as reactivity series. The metals lying above in the series are more reactive than the metals which lie below in the series.

On moving down the group, the reactivity of non-metals decreases.

For the given options:

Option 1:  

As, iodine is less reactive than bromine. It will not be able to displace bromine from its reaction. So the reaction will not occur.

Option 2:  

As, iron is less reactive than aluminium. It will not be able to displace aluminium from its reaction. So the reaction will not occur.

Option 3:  

As, nickel is more reactive than silver. It will easily displace silver from its reaction. So the reaction will occur.

Option 4:  

As, lead is more reactive than zinc. It will not be able to displace zinc from its reaction. So the reaction will occur.

Hence, the reactions that will occur are and

the dependant variable is spilled water

the independent is the heat

if you're asking what comes out, it's steam.