One molal solution is one that contains

We know that Molality$=\frac{\mathrm{No}. \mathrm{of} \mathrm{moles} \mathrm{of} \mathrm{solute}}{\mathrm{Mass} \mathrm{of} \mathrm{solvent}(\mathrm{in} \mathrm{kg})}$

$(1\mathrm{kg} =1000\mathrm{g} )$

So, Molality is $1$mole of the solute in $1000 \mathrm{g}$ of solvent.

Hence, option(a) is correct.