C₂H₄O₂ + O₂ ➞ CO₂ + H₂O

C2H4O2 + 2O2 ==> 2CO2 + 2H2O

Explanation:

Start with the carbons

C2H4O2 + O2 ==> 2CO2 + H2O There are 2 on the left, so you need 2 on the right.

Next deal with the hydrogens. You have 4 on the left so you have to make 4 on the right

C2H4O2 + O2 ==> 2CO2 + 2H2O

The oxygens are the real devil in this question. Be careful how you handle them. There are 2 * 2 = 4 from the CO2 and 2*1 = 2 from the water. The total is 6

Now you can't just put a 3 in front of the O2. There are 2 in the given chemical. So you don't need 6. You need 6 -2 = 4. But the oxygen is O2. You have to divide the 4 by 2 to get 2

C2H4O2 + 2O2 ==> 2CO2 + 2H2O  

And that's your answer.

the answer to your question is c

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.