You have the option of choosing what device respondents take your survey on: Desktop, Mobile, Tablet. 

Within your project builder, under the Respondent Targeting Tab- click the Advanced button:

This will expand the section where you can select or deselect these on the bottom of the targeting section. Simply un-click to exclude certain devices from entering your survey.

You can do the same thing in the targeting, as shown here, by selecting Device Type:

If you want only one specific platform to enter your project, select Device platform. For example, if you want only iPhone users, select iOS:

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