A $2\,\text{kg}$ block hangs vertically by a string and remains at rest. Taking $g = 10\,\text{m s}^{-2}$, what is the tension in the string?

If the block is at rest, acceleration is zero, so net force is zero. Therefore tension equals weight: $T = mg = 2\times 10 = 20\,\text{N}$. Hence option B.