Answer:
The correct answer is option d<u><em>. Speaking and understanding a language.</em></u>
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. <em>This is why </em><em>option d.</em><em> is the correct answer.</em>
<em>Also, this is why </em><em>option a. </em><em>is not correct, because</em> only <u>knowing grammar rules doesn't mean you know how to apply them with ease in a conversation. </u>People can know some grammar rules but don't know how to speak at all, because they don't know how to apply them, <u>as a fluent person do. </u>
On the other hand, when you start learning a language you get familiar with the culture and routines from people that speak it, as <em>option b.</em> stands out; and and therefore you learn that there are different ways of living, so you learn to be respectful of that, as<em> </em><em>option c.</em> says. <u>But this is a collateral learning</u> from <u>learning</u> a <u>language</u> and <u>not being fluent in it.</u>