(a)There are five permutations :
123 132 213 321 312
(b)There are 14 such permutations:
1234 1243 1324 1423 1432 2134 2143
3124 3214 4123 4132 4213 4312 4321
(c)Appendix A,Corollary A.10,shows that the number of permutations of n objects obtainable by a stack is the Catalan number
1 ( 2n )
n+1 n