Which one of the following shows a unique relationship with an insect that has coevolved with it and that is the only insect that can pollinate this tree?

Fig

Fig trees have a highly specialized mutualistic relationship with fig wasps. In many fig species, a particular wasp species is the exclusive pollinator, and the wasp depends on the fig for reproduction. This relationship is a classic example of coevolution. Mahua, sandalwood and silk cotton do not exhibit this same well-known, obligate fig-wasp-type pollination relationship. Therefore, option A is correct.