Final answer:
Increasing the spatial frequency enhances the number of line pairs per millimeter, directly affecting the resolution of an image. This means more detail can be resolved, but it does not affect the object size, number of photons, or light spread.
Step-by-step explanation:
The question pertains to the concept of spatial frequency, which is related to the detail or granularity of an image. Spatial frequency is measured in terms of the number of line pairs per millimeter (lp/mm). As the spatial frequency is increased, we are referring to increasing the number of line pairs within a given distance, which means more detail can be resolved.
This relates directly to the image's resolution. Therefore, when the quality factor called spatial frequency is increased, what increases is the number of line pairs per millimeter (option 3).
Planck's probability factor mentions that higher-frequency light has more energy, but this has no direct relation to spatial frequency in the context of image detail. Similarly, concepts like diffraction patterns from slits and numerical aperture affect resolution but do not directly answer the question related to spatial frequency.
It is important to note that increasing spatial frequency doesn't increase the object size, the number of photons, or the light spread; it enhances the resolution of an image by increasing the number of distinguishable line pairs per millimeter.
Therefore answer is 4. object size.