Which of the following pairs represent synonyms?

The pair of words "hard" and "difficult" represent synonyms because they both convey a similar meaning. When something is hard, it often requires a lot of effort or struggle to accomplish, while difficult also indicates that something is challenging or not easy to do. Since both terms can describe a situation or task that is not straightforward or requires substantial effort, they are interchangeable in many contexts. This shared meaning is essential in recognizing them as synonyms.

The other pairs do not share similarities in meaning. For instance, "beautiful" and "ugly" are opposites, while "dangerous" and "safe" represent opposing concepts of risk. Similarly, "quick" and "slow" describe contrasting speeds. Hence, only the first pair effectively demonstrates the relationship of synonymy.