b
\(\quad \quad \quad \quad \quad K _{4} Fe ( CN )_{6} \rightleftharpoons 4 K ^{+}+ Fe ( CN )_{6}^{4-}\)

Initial conc. \(1\, m\quad \quad \quad \quad \quad \quad 0\quad \quad \quad \quad 0\)

Final conc. \((1-0.4) \,m \quad \quad 4 \times 0.4 \quad \quad 0.4\, m\)

\(\quad \quad \quad \quad \quad \quad =0.6\,m \quad \quad \quad =1.6\,m\)

Effective molality \(=0.6+1.6+0.4=2.6 \,m\)

For same boiling point, the molality of another solution should also be \(2.6\, m\).

Now,\(18.1\) weight percent solution means \(18.1\, gm\) solute is present in \(100\, gm\) solution and hence, \((100-18.1=) 81.9 \,gm\) water.

\(\text { Now, } \quad 2.6=\frac{18.1 / M }{81.9 / 1000}\)

\(\therefore\) Molar mass of solute, \(M =85\)