Correct Answer: making a quick buck

Solution : The correct option is the third option.

Explanation:
Making a quick buck means engaging in activities to earn money quickly, often without much effort or consideration for potential risks. In the context of the sentence provided, it fits perfectly as it refers to the act of making money swiftly, which is highlighted as an easy but risky endeavour.

The meanings of the other options are as follows:

• In for a penny, in for a pound: This idiom means that if someone is involved in a situation, they are fully committed to it regardless of the consequences.
• Not fair and square: This phrase implies that something is not done in an honest or straightforward manner.
• Bigger fish to fry: This idiom means having more important or pressing matters to attend to.

Therefore, the correct answer is making a quick buck.