The correct answer is option 3: both (a) and (b). Metamorphism is a geological process that changes the structure and texture of rocks.

When rocks undergo metamorphism, their structure refers to the arrangement and orientation of minerals within the rock. This can change due to the application of heat and pressure during the metamorphic process.

The texture of a rock refers to the size, shape, and arrangement of its grains or crystals. Metamorphism can cause changes in the texture of rocks by recrystallizing existing minerals or forming new ones.

It is important to note that while metamorphism can alter the structure and texture of rocks, it usually does not significantly change their actual chemical composition. However, in some cases, chemical reactions may occur during metamorphism, leading to changes in the composition of the rocks.