Which of the following features of the student's interlanguage is most evidently still in development?

Causative constructions, while important, are often less frequently used in everyday communication compared to relative clauses. Intermediate and advanced ELLs may have had enough exposure to causative forms like "have" and "make" to employ them in appropriate contexts, suggesting that this feature could be more developed than relative clauses.

The use of contractions is generally one of the earlier aspects of language acquisition that ELLs grasp, as they frequently occur in spoken English. By the intermediate level, many students are usually comfortable with contractions such as "don't," "can't," and "it's," indicating that this area is likely less problematic than the use of relative clauses.

While verb tenses can be tricky, intermediate and advanced ELLs typically have a good grasp of basic tense usage, including past, present, and future forms. Their ability to manipulate verb tenses is often sufficient for effective communication, suggesting that this feature may not be as evidently in development compared to the more complex structure of relative clauses.