Correct Answer: C
Explanation:
Because "all workers" is plural, the pronoun that refers to it as its antecedent must also be plural. Thus, the correct version of the sentence is: "It is explicitly stated in the employee handbook that all workers must provide their own steel-toed shoes."
If the workers are made individual ("each worker") then the correct pronoun form would be "his or her," because the individual pronoun is required but the gender of the worker is not specified.