Final answer:
Motivation and attitude are the most important elements for effective second-language acquisition, trumping grammatical proficiency, cultural immersion, and instructional materials.
Step-by-step explanation:
The question about what is most important for effective second-language acquisition is a complex one, but research in linguistics and psychology suggests that motivation and attitude are critical factors. While grammatical proficiency is necessary for mastering a language, and cultural immersion can greatly enhance language learning and instructional materials are useful tools, it is ultimately the learner's motivation and attitude that determine their success. This is supported by various theories that state learning a new language is influenced by societal and cultural factors and that language acquisition happens naturally when the learner is highly motivated and positively engaged with the language and its culture.