D

Step 1: Light travels in a vacuum at a constant speed of approximately 299,792 kilometers per second (or about 300,000 km/s). This high velocity contributes significantly to the perception that light moves in straight lines.
Step 2: The phenomenon of light traveling in straight lines can be understood through the principle of rectilinear propagation, which states that light travels in straight lines in a uniform medium.
Step 3: While scattering and absorption can affect the path of light, in general situations (such as in clear air), light maintains its straight trajectory primarily due to its high velocity.
Step 4: Other options fail to justify the straight line path of light effectively. The small wavelength does not prevent bending due to scattering, and reflection doesn't necessarily lead to straight-line travel.
Therefore, the correct answer is D.