But thy eternal summer shall not fade,
nor lose possession of that fair thou ow'st,
nor shall death brag thou wander'st in his shade,
when in eternal lines to time thou grow'st
which statement best expresses the central idea of the third quatrain?
a. your beauty will never fade, because you will live forever in the lines of this poem.
b. love provides safety, so you should never wander from your beloved.
c. nature is more beautiful than a poem, but also more dangerous.
d. the beauty of summer fades when winter comes.
( not b )

The statement that best expresses the central idea of the third quatrain is A. your beauty will never fade, because you will live forever in the lines of this poem.
The poem is about not being forgotten - and if someone is eternalized in a poem, they will forever be alive and remembered. So will this woman, and her beauty forever be embodied in this poem which will exist forever, and thus will never be forgotten. 

The answers are:

Tyranny – democracy. The first is a form of government based on an absolute ruler, whereas democracy is the government of the people and for the people.

Discord – unison: The first noun refers to something that is not concordant with something else; the second one refers to something that is in accord with something other.

Dissentient – supporter: The first is one who disagrees on an issue; the second, one who back a certain issue.

Forgiveness – vengeance: the first noun refers to the capacity to show mercy or grant a pardon; the second refers to the will to seek retribution.

Obstinate – agreeable: the first one refers to being stubborn or inflexible; the second one to be willing to consent.

Obvious – cryptic: the first one refers to something that is clear and manifest; the second one to something obscure or enigmatic.