Registering to vote when a citizen turns 18 is NOT directly an example of a citizen participating in policy-making decisions.
Actually voting would be a way to participate in such decisions, both in terms of which candidates one votes for as well as which ballot initiatives (in referendum issues) the voter supports. The other answer choices all show the citizen participating in ways that express policy preferences. Campaigning for a particular candidate would be done to show support for policies supported by that candidate. Writing letters and demonstrating also are actions intending to shape public policy.
