Learn English – Difference between dozed off and slept

Bloat and bulge mean distended. They have a sense of heaviness and so are not appropriate for a balloon being filled with air or helium. If the balloon was being filled with something heavier like water, then those words might be appropriate.

A balloon will swell as it is being inflated with air. Swell means getting bigger.

Your examples are mixing two senses of the word challenging. The entry in Slovar-vocab is wrong when it states "stimulate" is a synonym of "challenge" when it means "something difficult".

1. The book I read stimulated me.
2. The book I read challenged me.

In sentence 1, the book is thought provoking or inspired interest in a topic.
In sentence 2, the language of the book was difficult and it took a lot of effort to read, or it forced me to confront my thinking about something and it was a little uncomfortable for me to realize that maybe I need to change my opinion or understanding. It's not clear from the context which meaning is the intended one.

One interesting thing to note is that while "stimulating" is sometimes listed as a synonym for "challenging", the opposite isn't usually true. They are only related through the word "provocative" which has two meanings:

1. Serving or tending to elicit a strong, often negative sentiment in another person; exasperating.
2. Serving or tending to excite, stimulate or arouse sexual interest.

"Stimulating" is provocative in a positive sense, but without necessarily the sexual connotation, like "exciting". "Challenging" can mean provocative in a negative sense, like "defiant" or "disturbing", but it usually means "difficult to do".