Answer:
The lines from the excerpt that are refrains are indeed:
- Do not go gentle into that good night;
- Rage, rage against the dying of the light
Explanation:
The word refrain comes from the Latin "refringere", which means "to repeat". A refrain, be it in poetry or music, is the line or lines that show up more than once. The types of poem that fixedly present a refrain are the villanelle, the virelay, and the sestina.
The poem "Do not go gentle into that good night", by Dylan Thomas (1914-1953), is a villanelle. That means it presents five tercets followed by a quatrain, and it has two refrains. The refrains are the bolded lines in the excerpt below:
Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.
Though wise men at their end know dark is right,
Because their words had forked no lightning they
Do not go gentle into that good night.
Good men, the last wave by, crying how bright
Their frail deeds might have danced in a green bay,
Rage, rage against the dying of the light.