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

Assuming you had to choose between:

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.

in mcculloch v. maryland (1819) the supreme court ruled that congress had implied powers under the necessary and proper clause of article i, section 8 of the constitution to create the second bank of the united states and that the state of maryland lacked the power to tax the bank.

gibbons v. ogden, (1824), was a landmark decision in which the supreme court of the united states held that the power to regulate interstate commerce, granted to congress by the commerce clause of the united states constitution, encompassed the power to regulate navigation.