The fastest growing group of americans in 2005 categorized themselves as

Multi-racial  Americans are the fastest growing group in 2005.

Explanation:

Multi-racial Americans are among the fastest-growing society of demographics in America. There are few young, tolerant, and growing at the fastest speed than other populations. As America is divided because of race, different social groups are against interracial marriages. However, some multiracial adults are proud of their mixed-race lives; they also feel that their human race has given them open to other diverse cultures. Because of this, a high number of population is proud to have a multi-racial background.

it foreshadows the theme of the paragraph and hints the topic that will be discussed in the paragraph!