Is it correct to write
happiness and prosperity are the result of diligent work
happiness and prosperity are the results of diligent work
You can use the singular or the plural depending on your intended meaning. If you mean that diligent work produces the combination of happiness and prosperity, then you can use the singular "result". If you mean that diligent hard work produces happiness and also produces prosperity, then you can use the plural "results". Both sentences are grammatical. The difference is subtle and conceptual.