AN important part of this problem is the fact that you're dealing with a molecular compound. That means that it will dissolve in aqueous solution without dissociating into ions.
An important consequence of that is the fact that the van't Hoff factor,
In simple words, one molecule dissolved, one molecule in solution.
Now, the equation that describes freezing-point depression looks like this
#color(blue)(DeltaT_f = i * K_f * b)" "#, where
The cryoscopic constant of water is equal to
Now, the freezing-point depression is defined as
#color(blue)(DeltaT_f = T_f^@ - T_f)" "#, where
In your case, the freezing-point depression will be
#DeltaT_f = 0^@"C"- (-0.104^@"C") = 0.104^@"C"#
So, all you have to do from this point on is to plug your values into this equation and solve for
#DeltaT_f = i * K_f * b implies b = (DeltaT_f)/(i * K_f)#
#b = (0.104 color(red)(cancel(color(black)(""^@"C"))))/(1 * 1.86 color(red)(cancel(color(black)(""^@"C"))) "kg mol"^(-1)) = color(green)("0.0559 mol kg"^(-1))#