Which phrase is most understandable without a hyphen?

Which phrase is most understandable without a hyphen?
small mammal studies
37  52.1%
labor saving devices
21  29.6%
stress producing circumstances
9  12.7%
hands on activities
4  5.6%

Number of Voters  :  71
First Vote  :  Thursday, September 03, 2026 12:55 PM
Last Vote  :  Thursday, October 01, 2026 09:33 AM