Voter turnout among 18-year-olds
a.
has been consistently among the highest in the nation.
b.
was higher immediately after passage of the 26th amendment, but then dropped significantly.
c.
was low at first, but gradually rose.
d.
demonstrates that this age group is more politically involved than are older americans.

18 year olds were given the right to vote with the 16th Amendment, and the turnout of 18 year olds just after this amendment was relatively high, but later dropped - answer b.

An informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.