One litre of water weighs $1$ kg. How many cubic millimetres of water will weigh $0.1$ gram?

The correct option is A $100$ cubic mm.
$1$ litre $=1000$ cubic cm of water weighs $1000$ g.
$\therefore 1000$ g is the weight of $(1000\times 1000)$ cubic mm ....$(\because 1$ cm $=10$ mm)
$\therefore 0.1$ g is the weight of $\frac{1000\times 1000}{1000}\times 0.1$ cubic mm $=100$ cu mm.