Let's define what an observation is:
An observation is a quantity or quality of something that you can see without making inference.
Let's apply this definition to our problem:
We are trying to find which is the observation between the three.
0. A. You conclude that the season is changing. - This would be ,inference,. You are "guessing" what ,will happen,, not something that ,has happened,. You don't know for sure that the season is changing.
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1. B. You hypothesize that the plant will grow 1 mm every day for a week. - This would also be ,inference,. The key word is ,hypothesize,; you are making an ,educated guess,.
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2. C. You measure a plant's height every day for a week. - This would be an ,observation, since you can ,quantitatively ,measure and ,observe, that plant's heigh every week.
Therefore, our best answer choice is C. You measure a plant's height every day for a week.