To import values for a dropdown/multiple-select questionnaire field via a source, you should use the special notation in your source as described here.
In a single field of an Excel/CSV file, insert values separated by a vertical line/pipe, for example "optionA|OptionB|OptionC".
Here is an example of the required format when importing multiple-selection options via a source:
Application | Normal text field | Multiselect text field |
"App A" | "normaltext" | "multitext1|multitext2|multitext3" |