The most likely observation BEFORE the experiment is noticing the number of tulip plants that sprouted late in a garden, this would give you a reason to do the experiment.
Just so you know, the 3 other options would all have to be done after the experiment because:
1) you cannot measure plant growth in response to different fertilisers before you start the experiment as the plants haven't been introduced to the fertilisers yet!
2) you cannot determine mass of product from a chemical reaction yet because if the experiment hasn't started yet the reaction is yet to take place!
3) the plants won't have had time to germinate yet!
