Concept:A concept map is a diagram that shows relationships among concepts and sub‑concepts in a non‑linear, networked way.
Explanation:Statement (a) says it is a complex network. This is true because concept maps use boxes and linking lines to show connections.
Statement (b) says it is a linear arrangement. This is false; concept maps are never linear.
Statement (c) says they can be used as diagnostic tools. This is true; they help identify learning difficulties.
Statement (d) says they help a teacher gain new insights into the nature of mathematics. This is true, as mapping concepts reveals relationships.
Thus the true statements are (a), (c) and (d).
Answer:D. (a), (c) and (d)