Final answer:
The most effective criticism to someone who doesn't work hard is 'You need to work harder.' It is concise, clear, and focused on the desired outcome of working harder.
Step-by-step explanation:
The most effective criticism to someone who doesn't work hard can be expressed in the sentence, 'You need to work harder.' This statement directly addresses the issue of not putting in enough effort and suggests that the person needs to increase their level of dedication. It is concise, clear, and focused on the desired outcome of working harder. Other sentences in the options, such as 'Stop slacking off or I'll fire you' and 'I think you're lazy,' may come across as harsh or judgmental, while the sentence 'You need to work eight hours every day and spend less time chatting' provides specific advice that may not be applicable to every situation.
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