We can compare US to France or Workers in US to workers in France, as they are logically comparable.
A. Unlike the United States, French workers have a 35 hour work week imposed by law on large firms
Reason to eliminate: Here US is being compared with French Workers.
B. Unlike workers in the United States, French workers enjoy a 35 hour work week, which was imposed by law on large firms
Correct Option
C. Unlike those in the United States, the workers in France have imposed by law a 35 hour work week on large firms
Reason to eliminate: The comparison is correct, but the statement is saying that the worked in France have imposed a 35 work week - Which is incorrect
D. In France, unlike the United States, the workers have imposed a 35 hour work week by law on large firms
Reason to eliminate: The comparison is correct, but the statement is saying that the worked in France have imposed a 35 work week - Which is incorrect
E. In France the workers, unlike the United States, have a 35 hour work week, which was imposed by law on large firms
Reason to eliminate: Workers or France is being compared to US country