Concept:The first language is learned naturally from birth by listening and imitating the speech around a child, without formal teaching of grammar rules.Explanation:Children pick up their first language from family and community in their early years. This learning is informal and based on daily exposure. As a result, the first language deeply reflects the local accent, dialect, and regional expressions of the area where the child grows up. These features become ingrained and often appear even when the child later learns another language. In contrast, mastery difficulty (option A) is not a defining trait; school environment (option B) influences later learning, not the first language itself; and the grammar of a second language (option D) has no bearing on the first language’s development.Answer:C. marked by the accent and regional expressions of the area where students grow up