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.