The phrase 'doing your bit' means:

The correct answer is Option A (Playing your part in the work of the world.).

Key Points:
• Contextual vocabulary and idiomatic phrases from the reading passage.
• Paragraph 3 explicitly explains the phrase in context: "...you must try to remember that you are helping to produce the things that other people need; you are 'doing your bit' and playing your part in the work of the world."
• Thus, 'doing your bit' means making your useful individual contribution / playing your part in the work of the world.
• Key Takeaway: 'Doing your bit' means contributing your share or playing your part in collective work.