The original sentence a is correct. If refers to government in general. I don't like sentence b, but I think it's OK in BrE. I'd prefer Governments should enact strong policies to ensure a good quality of life for their citizens.
Adding articles changes the meaning of the sentences. Your second two sentences are also correct; the definite article means that we're talking about specific policies in a and a specific government in b.
In the third pair of sentences, a is unchanged from the original. B is correct, and phrases the sentence another way that my AmE ears would prefer to hear it.
Student or Learner