The legal right to expatriation means that you have the right to

The legal right to expatriation means that you have the right to live in a country that you aren't a citizen of. Since, an expatriate is someone who lives (either temporarily or permanently) in a country other than that of their citizenship.

The legal right to expatriation means that you have the right to renounce your citizenship. The right to expatriate has two forms, one is the right of the citizen to sever his ties with the state and the other one is the ability of the state to sever its ties with the citizens. American citizenship can be lost through acts like military desertion, failure to pay taxes, acceptance of any citizenship from other countries, and even a woman's marriage to a foreigner. It was ruled that a woman marrying a foreigner loses her U.S. citizenship while the marriage is in effect but resumes citizenship when the marriage is dissolved.

I’m pretty sure that the answer is b.