What is one characteristic of a sample of matter that indicates it is a pure substance?

One characteristic of a sample of matter that indicates it is a pure substance is the homogeneity of the molecular structure. If the structure is the same all throughout such as liquid water, it is said to be homogenous.

If the composition of a sample is fixed, the sample is a pure substance. According to the law of definite proportions (constant composition), components of a pure substance must be in definite ratios regardless of the source.

scattering of light by tiny particles in air to produce the colors we see on sky, is dependent on the following factors:

(i) wavelength of light

(ii) size of the scattering particle  

(iii) distance traveled by light through the atmosphere.

according to rayleigh's law of scattering, the amount of scattering is inversely proportional to the fourth power of its wavelength. shorter the wavelength, greater is its scattering. hence violet/blue light is scattered more than red.

sun's light is a composite light ( visible light) with wavelengths in the range 400 nm (violet) to 700 nm (red). as sunlight passes through our atmosphere, the molecules of air scatter light according to their wavelengths. during sunrise and sunset, the sun's position is below the horizon. hence light from the sun travels a greater distance in the atmosphere when compared to its position a noon.

as sunlight enters the earth's atmosphere, blue light is scattered away and at the end of its journey in the sky, only red and orange wavelengths remain in the beam.

hence at sunrise and sunset, the sky appears reddish orange.

i think the answer would be reserve.