Question

in order to vote, what must a person do prior to election day?
obtain a seal on their registration card
be over 21 years of age
register with the board of elections

Explanation:

In most electoral systems, the prerequisite to vote is formal registration with the relevant election authority before Election Day. A seal on a registration card is not a universal or standard prior step, and the minimum voting age in many regions (like the US) is 18, not 21.

Answer:

register with the board of elections