Somaclones can be defined as:

The correct answer is Option (2) → They are genetically identical to the original plant.

In tissue culture, a whole plant can be grown from any part of a plant (an explant) on a nutrient medium; this power of a cell is totipotency. Plants raised this way in thousands (micropropagation) are copies of the plant they came from.

Option (1) They have the capacity to generate a whole plant from any explant. — Incorrect. This describes totipotency, the property that makes tissue culture possible, not somaclones.

Option (2) They are genetically identical to the original plant. — Correct. Plants made by tissue culture are genetically identical to the original plant; such plants are somaclones.

Option (3) They are genetically modified pest resistant plants. — Incorrect. These are transgenic plants, such as Bt cotton, made by putting in a foreign gene.

Option (4) They are the plants, infected with virus. — Incorrect. It is the opposite: tissue culture of the meristem is used to get virus-free plants from infected ones.

NCERT Extract:
Biotechnological Applications in Agriculture – “… By application of these methods it is possible to achieve propagation of a large number of plants in very short durations. This method of producing thousands of plants through tissue culture is called micro-propagation. Each of these plants will be genetically identical to the original plant from which they were grown, i.e., they are somaclones.”