When congress passed the fugitive slave law in 1850, harriet beecher stowe started a serial novel about slavery. which of these phrases is an adjective phrase?

The phrase which is an adjective phrase in this sentence is about slavery.
Adjective phrases act as simple adjectives - they describe the noun they are referring to. So, what kind of a serial novel? About slavery. This phrase modifies the noun novel further.
In 1850 is an adverbial phrase, when Congress passed the Fugitive Slave Law is not a phrase at all but a dependent clause, and started a serial novel is a fragment, not a phrase.