A. The results of the experiment showed a quantifiable decrease in oxygen levels.
Explanation:
The chosen sentence possesses a formal register with technical vocabulary related to science, which can be understood by professional scientists. For instance, the words <em>quantifiable </em>and <em>decrease </em>have a more academic style. Besides, the writing is impersonal and written in the third person.
The rest of the options are incorrect because they have contractions and they are too informal. Thus, their vocabulary contain casual expressions which do not follow the conventions of an academic style.
