2. All my sisters are doctors. I am a teacher.
(join using whereas)

All of my sisters are doctors, whereas I am a teacher.

Explanation:

The given two sentences can be joined using 'whereas' in the form of a complex sentence as mentioned above. It is a conjunction that is employed at the beginning of the subordinate clause('whereas..teacher') to indicate or throw light upon a significant difference between two ideas or to state facts. In the given sentence, the contrast between 'my sisters' and 'I' have been shown which is both a fact and the difference as well. Thus, the final sentence reads as:

'All of my sisters are doctors, whereas I am a teacher.'

The answer is D.

Explanation:

All of the other statements can be said on their own. D would sound very strange in a normal conversation on its own/as it is.