D

Step 1: Understand that a magnet can lose its magnetic properties through a process called demagnetization.
Step 2: When a magnet is subjected to a strong enough reverse magnetic field, its magnetic domains can be reoriented such that they are no longer aligned, effectively demagnetizing the material.
Step 3: While options A (breaking into pieces), B (heating slightly), and C (dropping in ice cold water) may affect magnetic properties, they do not necessarily result in complete demagnetization.
Conclusion: A reverse field of appropriate strength (Option D) is the most effective method for complete demagnetization.
Therefore, the correct answer is D.