Some words don't really have good synonyms -- and that's OK. 

Three things you can do with them in an IELTS answer are: 

- repeat the word (nothing wrong with some repetitions); epeat the w

- use ellipsis; 

- use substitution. 

And here two more things you can do. 

Let's use this topic as a source of examples: "Advertisements are becoming more and more common in our daily lives. Is it a positive or negative development?"

How can you avoid repeating "advertisement"? ("Advert" and "ad" are not it.)

1️⃣ Take one step down

What are some specific examples of advertisements? 

- a commercial (only on radio, TV, or podcast)

- a banner (only for web pages) 

- a host-read / host-read segment (when, say, an influencer, reads a script about a product on their show) 

- a brand ambassador (a person, usually a celebrity, who represents the brand) 

Of course, these words have specific meanings, so you can't just use them interchangeably with "advertisement," but they will help you avoid repetitions and fill your essay with topic-specific lexis. 

2️⃣ Take one step up 

What might you call a lot of ads combined? 

- marketing campaign 

- promotion 

- (celebrity) endorsement 

- publicity

- media content 

- sponsored content 

Again, these words are not direct substitutes for "advertisement," but they will help. 

After all, your goal is not to use synonyms or even avoid repetitions -- your goal is to communicate meaning.