Answer
a. The smallest value of $A$ for which there are two stable nuclei is $A = 36$. The stable nuclei are $^{36}$S and $^{36}$Ar.
b. The value of $A$ less than this no stable nuclei is $A = 4$.
Work Step by Step
a. In question they have not asked for two stable nuclei with lowest mass numbers. However, in that case, it would be $^{4}$He and $^{12}$C . They have asked for the lowest value of $A$ for which we get two stable nuclei. Checking the list of stable nuclei, it is $A = 36$ and the stable nuclei are $^{36}$S and $^{36}$Ar.
b. The value of $A$ less than this no stable nuclei is $A = 4$. The nucleus is $^{4}$He . From the curve of nuclear stability we see that there is no stable nucleus below $A = 4$.
