Final answer:
The time to develop fluency in a second-language learning environment generally takes 5-7 years, considering the critical period for effective language acquisition is during early childhood and tends to decrease with age.
Step-by-step explanation:
In an optimal second-language learning environment, the time it takes for most students to develop fluency can vary widely. While young children have a biological predisposition to acquire language and can do so with relative ease, the ability to learn a second language diminishes with age. Some research points to a critical period in early childhood during which language acquisition is most effective. That said, for older students and adults, gaining fluency in a second language can take several years of study and practice. The answer, according to general observations and studies, would fall into the range of 5-7 years, which coincides with option c) 5-7 years. This timeframe would allow for the necessary immersion in the second language, proper attention to vocabulary, grammar, and pronunciation, as well as cultural proficiency.