In the IELTS Speaking exam you need to show that you have a broad vocabulary.
In English there are many words that have the same general meaning; however, they may have subtly different definitions.
Example: challenge and obstacle can both refer to a difficulty. However, if you use the word challenge, you imply that you may be able to overcome the difficulty. If you use obstacle, you imply that the difficulty is harder to overcome.
Learn to distinguish these subtle differences of meaning to express yourself precisely.