Which one of the following is not decidable?

$$\sum { = \left\{ {a,b} \right\},\,\,} $$ which one of the following sets is not countable.

Which one of the following regular expressions over $$\left\{ {0,\,\,1} \right\}$$ denotes the set of all strings not containing $$100$$ as substring?

Which of the following languages over $$\left\{ {a,b,c} \right\}$$ is accepted by Deterministic push down automata?