It is important for students to improve their skills

Solution

The phrase "skills in mathematics" correctly uses the preposition "in" to indicate the subject area where students need to improve their skills. The preposition "in" is appropriate for denoting the domain or field of expertise. The other options—"of," "under," and "on"—do not fit the context: "of" is used to show possession or association, not for indicating the subject area. "under" implies a different context, often indicating something beneath or subject to authority. "on" typically indicates the topic of discussion or focus but is less precise than "in" for describing a field of study. Thus, "in" is the most suitable preposition for the given context.