Option 1 states that zinc is costlier than tin, but this is not the reason why food cans are coated with tin instead of zinc. The cost of the metal does not determine its suitability for coating food cans.

Option 2 states that zinc has a higher melting point than tin. While this is true, it is not the reason why food cans are coated with tin. The melting point of the metal is not a factor in the decision to use tin instead of zinc.

Option 3 states that zinc is more reactive than tin. This is the correct reason why food cans are coated with tin instead of zinc. Zinc is more reactive, meaning it is more likely to react with the food or the environment, potentially causing contamination. Tin is less reactive, making it a safer choice for coating food cans.

Option 4 states that tin is more reactive than zinc. This is incorrect. It is actually the opposite, with zinc being more reactive than tin.

In conclusion, the correct answer is option 3 because zinc is more reactive than tin and can potentially cause contamination if used as a coating for food cans.