B

Step 1: Understanding the factors affecting light entry in a camera. The amount of light entering is primarily influenced by the diameter of the objective lens and the focal length due to the geometric formulation of light collection.
Step 2: The aperture of the camera, which determines how much light enters, is actually related to the focal length and the diameter of the lens according to the formula for the f-stop (aperture), which is given as:
$$ f ext{-stop} = \frac{F}{D} $$
where F is the focal length and D is the diameter of the lens. The larger the diameter relative to the focal length, the more light can enter, leading to a higher light-gathering ability.
Step 3: Among the options, the product of focal length and diameter directly influences the quantity of light due to their combined effect on the aperture.
Therefore, the correct answer is B.