Read the dictionary entry.

brood (brood)
noun
1. babies such as birds hatched at one time
2. a kind or group
verb
3. sitting on eggs to make them hatch
4. to warm or protect with the body or wings
5. to think hard or consider
6. to dwell or worry about something

which meaning of brood best defines the underlined word in the paragraph?

when mark has to make a big decision, he will agonize over it for days. it bothers him so much that he loses sleep and gets headaches. but when he comes to a conclusion, he's all smiles again.

a.
1. babies such as birds hatched at one time

b.
2. a kind or group

c.
4. to warm or protect with the body or wings

d.
6. to dwell or worry about something