Democratic lawmakers in Connecticut introduced legislation Thursday that would create a public option for health-care coverage, allowing individuals and small businesses to buy insurance through the state. Under the proposal, Connecticut would create multiple plans that small businesses and their employees could buy starting in July, leveraging the state’s existing purchasing power to lower costs. And in 2021 Connecticut would begin offering a new health-insurance option that any state resident would be able to join. The public insurance would offer coverage options for small businesses and individuals who don’t qualify for subsidies through the Affordable Care Act, officials said.