Light exhibits the Doppler effect. What does this tell us

The correct answer is option C, that is, a chemical indicator, and the beaker is green because a little CO2 is present.

The bromothymol blue refers to a pH indicator. It is mainly used in the processes that need the determination of substances, which would exhibit a relatively neutral pH. A common application of bromothymol blue is determining the existence of carbonic acid in a liquid.  

The bromothymol blue is an indicator, which turns from blue to green to yellow as the solution becomes more acidic due to the formation of carbonic acid.  

In the given case, the beaker becomes green in the presence of bromothymol blue because maybe there is some presence of carbon dioxide in the beaker, and as more formation of carbonic acid takes place, the solution in the beaker would turn more into yellow.