Words that are similar in meaning to each other are known as 'synonyms', and words that are quite unlike each other in terms of meaning are known as 'antonyms'. In this case, we have to find the word which IS a synonym of the word 'arduous', which means something that requires a lot of effort. Options A), B) and D) are all unrelated to not just each other, but also the word 'arduous' itself, and are hence ruled out - artistic refers to something that is creative; painful refers to something that causes pain and lastly rapid signified a fast speed or pace. This leaves us with Option C) - 'Laborious', which is the correct answer. 'Laborious' itself refers to something that requires a lot of effort.