Question

Labor is a _____ used to produce goods and services. salary reference human resource

Answer 1

Labor is a Human Resource used to produce goods and services. 

Answer 2

The word "labor" is roughly synonymous with "work" - and especially hard work, or physical work.
"Salary" is compensation for work- but not work itself.
Work is also not a reference, but far from it.
Work, however, is a human resource - and so it labour.