Verified answer

The correct sentences are:

There are four kinds of pollution in Hong Kong. Do you know what they are?

(Industrial pollution, traffic pollution, noise pollution and light pollution?)

Do you know who Class 2C's English teacher is? - this would be the most natural sentence in ordinary speech, but it is not 'good' English. The correct sentence is:

Do you know who teaches English to Class 2C?

"Do you know what they are?"

"Do you know who the teacher of Class 2C is?"

But: "Who is the teacher of Class 2C?" (Without 'Do you know') Both those forms are valid in British English.