Actually, this is what the question instructions are for!
While it is certainly possible to add a line break in Survey Solutions, check if you really need one. Perhaps, you are trying to bunch together some unrelated texts just because they appeared sequentially on paper? See also the relevant discussion in the Kobo forum, where the users are similarly confused on how line breaks can be included into question texts:
and the advice from Stephane Aloo.
Do check the documentation first. Please, go to the documentation site:
and type Line break in the search line in the site’s header. The very first result that comes up will resolve your doubts.
Finally, if you are reporting a bug, that the intended line break included according to the documentation is not being rendered, please describe clearly the example that can reproduce the issue.