Concept:The line expresses the idea of a love that is constant, faithful, and lasts for a whole lifetime.
Explanation:Here, "ne'er alter" means "never change."
The phrase "till thy sweet life end" means "until your life comes to an end."
Thus, the speaker is wishing that the love in question stays exactly the same from now until death.
Such love does not shift, fade, or disappear with time.
This directly opposes the meaning of a "fickle love," which changes quickly and easily.
The option "undying love" may appear attractive, but the expression is limited to the span of a person's life, not love that continues after death.
Therefore, the best choice is "enduring love," meaning love that remains steady and long-lasting throughout life.
Answer:A. an enduring love