Which of the following strategies are not suitable for students with low achievement in mathematics ?

Correct option is C

(a) Using activities centered on students' interest: This is highly suitable! Engaging activities motivate, build connections, and make learning enjoyable.
(b) Reducing the number of practice problems: This is also suitable. Fewer problems reduce overwhelm and allow deeper focus on each one.
(c) Assigning only the most difficult problem: This is NOT suitable. Starting with difficult problems without building foundational understanding can be discouraging and counterproductive.
(d) Allocating extra time for teacher-guided practice: This is suitable! Additional targeted guidance directly addresses understanding gaps and strengthens skills.
Therefore, the unsuitable strategy for low-achieving students is (c) Assigning only the most difficult problem. It's crucial to build confidence and understanding progressively, not start with advanced challenges.