There are 3 prepositional phrases: for civil rights, by means, and of protest. As a general rule the object of the preposition follows the preposition and is always a noun. So in the phrase for civil rights the object would be rights (civil is an adjective describing rights). The object of "by means" is means. The object of "of protest"
some activists for civil rights registered their feeling by means of protest. what is the object of preposition by means of