The half-life of a radioactive isotope is three hours. If the initial mass of the isotope were 256 g, the mass of it remaining undercayed after 18 hours would be

It is given in the question that, No=256g, T= 18hours and t1/2= 3hours. Now, we will use the formula and calculate the amount of undecayed isotope left. So, the mass of the isotope left undecayed after 18hours is 4g. 

18 hour implies six half-lives. Hence, the mass remaining undecayed will be

$m=\frac{256}{2^6}=4.0\mathrm{g}$

Hence, the answer of the given question is option (B)