Option 1: This option suggests that the pressure exerted by the knife increases with an increase in bluntness. However, this is incorrect. In reality, a blunt knife actually decreases the pressure exerted for a given force.

Option 2: This option states that a sharp edge decreases the pressure exerted by the knife for a given force. This is partially correct. A sharp edge reduces the surface area over which the force is applied, therefore decreasing the pressure. However, the sharpness of the knife is not directly related to the difficulty of cutting with a blunt knife.

Option 3: This option correctly explains that a blunt knife decreases the pressure for a given force. When a knife is blunt, the force is distributed over a larger surface area, resulting in lower pressure. This makes it more challenging to cut through objects.

Option 4: This option suggests that a blunt knife decreases the area of intersection. However, this is incorrect. A blunt knife would typically have a larger area of intersection with the object being cut due to its reduced sharpness.

Therefore, the correct answer is option 3. A blunt knife decreases the pressure for a given force, making it difficult to cut things efficiently.