Step 1: Understand the context.
The sentence expresses an expectation that the person has understood an important life lesson: failures lead to success. The blank requires a verb that indicates understanding or awareness.
Step 2: Analyze the options.
Known is grammatically incorrect here because the correct structure would be "have known", but it does not fit naturally in this context.
Felt refers to emotion rather than understanding a concept.
Decided means making a choice, which is not relevant here.
Realised means understood something clearly, especially after experience. This perfectly fits the sentence.
Step 3: Conclusion.
Therefore, the correct word is realised, as it conveys understanding gained over time.