Solution:

A cone clutch is a type of mechanical device used in automobiles to engage and disengage power transmission between the engine and transmission system. It consists of two conical surfaces, one attached to the driving shaft (input shaft) and the other to the driven shaft (output shaft). When these conical surfaces are pressed together, friction is generated, allowing torque to be transmitted between the shafts.

Now, the disadvantage of the cone clutch, particularly when the cone angle is less than 20°, lies in the difficulty to disengage the clutch. The cone angle refers to the angle formed by the tapering surfaces of the clutch components. When this angle is too small, it reduces the effective diameter of the clutch, making it harder to separate the clutch plates. As a result, more force is required to disengage the clutch, leading to operational difficulties and potential damage to the clutch components over time.

Other options are incorrect because:
Option B: Being silent in operation is not a disadvantage but rather a desirable characteristic of a clutch system.
Option C: The normal force on the contact surface being larger than the axial force is not a typical disadvantage of cone clutches. It's more of a design consideration that needs to be managed to ensure proper functionality.
Option D: Transmitting the same torque for the same size as the plate clutch is not a disadvantage. In fact, it can be considered an advantage in terms of efficiency and space-saving design.

In summary, the correct answer is Option A because a small cone angle can indeed make it difficult to disengage the clutch, which is a notable disadvantage of cone clutches.