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.

Wild men who caught and sang the sun in flight,
And learn, too late, they grieved it on its way,
Do not go gentle into that good night.

Grave men, near death, who see with blinding sight
Blind eyes could blaze like meteors and be gay,
Rage, rage against the dying of the light.

And you, my father, there on the sad height,
Curse, bless, me now with your fierce tears, I pray.
Do not go gentle into that good night.
Rage, rage against the dying of the light.

How might these people have “sang the sun in flight” and then “grieved” it?

The way in which the people might have “sang the sun in flight” and then “grieved” it is A. They "sang the sun in flight" by sending it off in grand style. They praised it when they had it, and so when they recognized they no longer had a strong grasp on it, they "lamented" its loss, just as when they had the sun, they praised it but imagined it would be theirs forever, and when it was gone, they grieved what power they had.

What is a Stanza?

This refers to the way a poem is arranged and which the lines are written in a particular order.

With this in mind and from the given text, we can see that in the fourth stanza, the people sing the sun in flight and then grieved because as they could no longer grasp it, they were sad and grieved for their loss and then sent it off in grand style.