Hi Sasha, The best answer would be "regarded" because of the word "curiosity". It was the first time the students saw their new teacher and they were trying to form an opinion of her (e.g. Was she going to be friendly, professionalism or helpful? Was she knowledgeable and competent in her teaching? What was her background?). So, the students were curious about her and perhaps looking at her demeanour to form an opinion of her. We often use phrases such as "regard (someone) with (a feeling/attitude)" such as suspicion, favour, fondness and so on. As I mentioned earlier in your other post, we "observe" when we look purposefully in order to learn something. This word would be more suitable if the sentence were: The students "observed" how their new chemistry teacher perform titration to determine the concentration of a solvent." "Watched" can also be used here but the meaning of discovery and learning may not be so obvious. Lastly, "watched" would be more suitable if your sentence were: The students "watched" (not "observed" nor "regarded") in delight when their 75-year-old chemistry teacher jived to the beat before doing a somersault. I hope this helps.

Watched The other choices are too formal and unnatural. :-)

Regarded would be my first choice. Watched is ok. Observed is not the correct word in this sentence.

Watched is natural and common. Observed is natural, less common, and indicates "watching carefully." Regarded doesn't seem natural except for a few fixed phrases like "regard with suspicion, with curiousity, with malice." The students watched the teacher perform the chemistry experiment. (natural) The students observed the teacher perform the chemistry experiment. (natural) The teacher walked into the room on crutches. The students looked at the teacher with curiosity. (natural) The students watched the teacher with curiosity. (natural) The students observed the teacher with curiosity. (natural) The students regarded the teacher with curiosity. (natural)

'watched', because this is a simple action - they wanted to know what he/she was doing at that moment. 'observed' is strange if the students are in the classroom! and 'regarded' is not used like this in English - it sounds like French