A discrete quantity of energy proportional in magnitude to the frequency of the radiation it represents
It's D your choice as commas, split an explanation while the conjunction 'but' links two sentences. No need to separate them by using a semicolon.
Immigration should be regulated because only legal immigrants contribute to the economy, skilled immigrants fill in-demand job vacancies and legal migration provides a safe option for at-risk groups of people to immigrate to the United States.
