Specify that a response is not required

Allow participants to proceed through your question without entering an answer.

By default, participants must enter an answer before proceeding.

For Short Answer, Long Answer, Numeric, Rank Order, Multiple Choice Grid, Single Choice Grid, and Choice questions:
In the Edit Pane, under Additional Options, clear the Response required check box.
For Allocation questions:
At the bottom of the Edit Pane, clear the Must use full allocation amount check box.