Answer:
The correct answer is option d. Speaking and understanding a language.
Explanation:
When somebody is fluent in a language it means they have the ability to express themselves spontaneously because they know some grammar rules and, more importantly, they know HOW TO APPLY THEM IN A CONVERSATION, therefore people understand what they say in that language. This is why option d. is the correct answer.
Also, this is why option a. is not correct, because only knowing grammar rules doesn't mean you know how to apply them with ease in a conversation. People can know some grammar rules but don't know how to speak at all, because they don't know how to apply them, as a fluent person do.
On the other hand, when you start learning a language you get familiar with the culture and routines from people that speak it, as option b. stands out; and and therefore you learn that there are different ways of living, so you learn to be respectful of that, as option c. says. But this is a collateral learning from learning a language and not being fluent in it.
