The code fragment requires an operation that will result in the output 'PEEK: Unix'. The Stream API methods 'findAny()' and 'findFirst()' both return an Optional of an element in the stream, triggering the intermediate operations including 'peek'. Therefore, inserting 'findAny()' or 'findFirst()' at line n1 will result in the expected output. Other methods like 'anyMatch()', 'allMatch()', and 'noneMatch()' require a Predicate and return a boolean, which would not trigger the 'peek' method in the same way.