In which of the following does the voter get a chance to choose from only democratic candidates?

Assuming you had to choose between:

Democratic Primary Election Iowa Caucus Democratic National Convention None of the above

The answer is: the Democratic Primary Election.

You could also add the Democratic National Convention, but only pledged delegates get to vote for their Presidential nominee ( + Vice President) in these elections, and not all citizens.

In the case of the Democratic Primary Election, however, all citizens vote for the candidate they want to run as a Democrat in the general election. Winning candidates then send their nominees to the Democratic National Convention.