The correct answer to which line from "Mutability” can be seen as a reflection of the idea suggested in "A Defence of Poetry" is "Nought may endure but Mutability".
This is the poem's last line, and it summarizes an idea conveyed throughout the whole poem. When Shelley says that nothing may remain in existence, nothing may last or survive but mutability, he means that the only thing that can possibly last is change, movement, or the tendency to change. Thus, this line can also be seen as an example of the poet's idea, suggested first in "A Defence of Poetry”, that the only thing that lasts is change.
The answer couldn't be "Or like forgotten lyres, whose dissonant strings", because the continuity to this line is what conveys the idea that the lyres change according to their dissonant strings, that vary and give many different responses depending on how they are played. Just this line alone doesn't take the reader to the understanding of change as the only thing that lasts. Therefore, this line couldn't be the correct answer.
The answer couldn't be "One mood or modulation like the last", because, just by this line, it can be inferred that there is such a thing as mood or modulation like the last, and that is opposed from the idea of change as the only thing that lasts. If the mood or modulation is just like the last, then there was no change. Thus, this line couldn't be the correct answer.
The answer couldn't be "We feel, conceive or reason, laugh or weep", because, although this line gives an idea of human states changing from one to another, it doesn't convey that this could be the only thing that lasts. It just shows that changes exist, but not that they are the only thing remaining. That is why this couldn't be the correct answer.