The correct answer is d. porphyritic texture.
Porphyritic texture is an interesting and unique feature observed in some igneous rocks. This texture is characterized by two distinct sizes of mineral grains: larger crystals, known as phenocrysts, embedded in a finer-grained matrix or groundmass made up of smaller crystals. The difference in size typically indicates that the rock underwent two stages of cooling.
In contrast, the other options describe different textures: a. phaneritic texture features crystals of similar size that are generally visible to the naked eye, b. aphanitic texture consists of crystals that are too small to see without a microscope, and c. pegmatitic texture refers to extremely large crystals, usually greater than 2.5 cm in size, found in a very coarse-grained rock. Therefore, the defining characteristic of porphyritic texture is the presence of both large and small mineral sizes within the same rock, making it distinct from the other textures.