What is the deeper reason for keeping silver bromide in a dark coloured bottle?

Correct answer: To prevent its decomposition by light energy

Silver bromide is photosensitive. Light supplies energy that can cause it to decompose, producing silver and bromine; this property is related to photochemical reactions and was used in photographic materials. A dark or amber bottle absorbs or blocks much of the incident light, reducing decomposition during storage. Therefore A is correct; the bottle is not intended to promote burning, dissolving, or mass gain from air.