A political division, not a ranking by size.
The Basic Law’s preamble names the Länder individually. A state’s size does not affect the count: each counts once. A Land is a political unit with its own state institutions.
A common mistake is to treat Berlin, Bremen or Hamburg only as city names and omit them. They are also Länder. Bremen includes the cities of Bremen and Bremerhaven but still counts as one federal state.
The other three answers
The three wrong numbers do not match the complete present list of Länder.
14
Fourteen is too few. Omitting city-states means failing to count all the Länder.
15
Fifteen leaves one Land out. Even relatively small Bremen counts as a state in its own right.
17
Seventeen is too many. An additional city or region does not become a federal state merely because it has a name.
Do not count Länder and their cities twice.
A Land can contain many municipalities. Municipalities are not added to the state count. An administrative district is not an additional federal state either.
Today’s number describes the present Federal Republic. Historical maps can show different borders or divisions. This question concerns the current list, not a map from an earlier period.
Preamble · Basic Law ↗16 Länder—including the three city-states.
Keep the count at the same political level throughout.
Three words for this question
- das Bundeslandfederal state
- A constituent state of the Federal Republic of Germany.
- der Stadtstaatcity-state
- A state whose territory is predominantly urban.
- die Gemeindemunicipality
- A local political unit within a Land.