A thesaurus is a tool which can help you find synonyms for particular words, and there may be lots to choose from. However, it is important that any synonym you choose means the same in context as the original word or phrase.
For example, if we take the word “dying”. The literal meaning of this word is something that is reaching the end of its life. But in the exam, you might be asked to explain what the writer means by the word “dying” in a sentence such as:
On the other side, there was a streetlamp with a dying bulb.
Here, the examiner would be looking for an explanation of the word “dying” appropriate to the context in which it has been used. So, in this case, the bulb is becoming fainter and dimmer, so the appropriate synonyms would be “fading” or “dimming”.
This is why it is always important to consider the context in which a word or phrase has been used in order to explain it.