The Fifth Amendment has an explicit requirement that the Federal Government not deprive individuals of "life, liberty, or property," without due process of the law and an implicit guarantee that each person receive equal protection of the laws.
The Fourteenth Amendment explicitly prohibits states from violating an individual's rights of due process and equal protection. Equal protection limits the State and Federal governments' power to discriminate in their employment practices by treating employees, former employees, or job applicants unequally because of membership in a group, like a race, religion or sex. Due process protection requires that employees have a fair procedural process before they are terminated if the termination is related to a "liberty," like the right to free speech, or a property interest
Option D: Absolutely. Anyone can discriminate against anyone, as long as it isn't being done by the government, or a monopoly (which is usually propped up by the government). However, prejudice is free, but discrimination has costs, in that the lost revenue will cause people to re-think their actions UNLESS the majority agrees with them.
Technically, they already are. However, you can't treat someone differently because of that. Heteros can't refuse to bake a wedding cake for a queer couple the same way a vegan can't refuse to serve steak at a restaurant. Honestly, I would rant forever about how awful queerphobia is and how minorities are being mistreated but I think this specific topic has been debated enough