Amutagen is

a) a living thing that has undergone a mutation.
b) an agent that causes a mutation in dna.
c) a mutation that has affected one gene.
d) a chemical that can poison living cells.

The correct answer is B) An agent that causes a mutation in the DNA.

Mutagen is a chemical or a physical agent that brings change in the genetic material of an individual and therefore increases the chances of mutation(permanent change in the genetic material).

Therefore, it is an agent that causes mutation in the DNA.

Example of mutagen- Ionizing radiation (like X- rays) , UV radiations.

answer and explanation;

volcanic rocks are igneous rocks which form from lava, molten rock which has been thrust out of a volcano onto the surface of a rocky body such as an asteroid, planet, or dwarf planet.  

plutonic rocks are igneous rocks which form from subterranean chambers of molten rock or magma. plutonic rocks make up the base of continental crust as well as oceanic crust. their slow rate of formation causes them to be coarse grained since there is plenty of time for large crystals to form before the magma solidifies into rock.

the textural difference is that plutonic rocks are more coarse grained than volcanic rocks. plutonic rocks are coarse grained while volcanic are fine grained ; glassey or fragmental. since fine and coarse are adjectives the decision as to whether a rock is fine or coarse grained is generally made on the bases of whether or not the individual crystals can be seen in most if the rock.