To determine which feature distinguishes a centre punch from a prick punch, we need to consider their construction and purpose.

Centre Punch:

• Used to make a large and slightly deeper indentation on metal surfaces to guide the drill tip for accurate drilling.
• Characterized by a blunter point angle, usually around 90 degrees. This helps to prevent slipping off when guiding a drill.

Prick Punch:

• Designed for making small and precise marks as a guide for other tools or detailed measurements in layout work.
• Has a sharper point angle, typically around 30 to 60 degrees, to make finer markings.

Now, let's examine the options:

1. Knurled hand grip: Both centre punch and prick punch can have a knurled hand grip for better control.
2. Blunter point angle: The centre punch indeed has a blunter point angle compared to the prick punch. This is the primary distinguishing feature concerning their construction.
3. Steel material body: Both types of punches are generally made of steel to endure the force applied while punching.
4. Longer overall length: The lengths of these tools can vary based on manufacturer preferences, and it's not a distinguishing feature based on construction.

Based on the explanation above, the correct answer is that the centre punch is distinguished by a blunter point angle compared to a prick punch. This design serves the function of guiding drills by providing a larger starting hole.