D

Step 1: Understand the concept of continuous spectra and absorption lines. A continuous spectrum is emitted by hot, dense objects, producing a smooth range of wavelengths without any gaps. However, when cooler gases or vapours are present in the path of this light, they can absorb specific wavelengths of light, leading to dark lines in the continuous spectrum.

Step 2: Analyze the options:
- Option A: Defects in the observing instrument would not cause missing lines but may cause distortion or errors in the spectrum observed.
- Option B: Absence of elements in the light source would not create dark lines; rather, it would lead to a continuous spectrum without lines associated with those elements.
- Option C: Presence of hot vapours could produce an emission spectrum instead of creating missing lines.
- Option D: Presence of cool vapours around the light source can absorb certain wavelengths of light emitted by the source, resulting in missing lines in the continuous spectrum.

Conclusion: The correct interpretation aligns with Option D, as cooler vapours around the light source lead to absorption lines in a continuous spectrum.