Neckar-Odenwald-Kreis is in Baden-Württemberg.

Assign the district to the correct level of government.

A district name identifies an area containing several municipalities and a district administration. For this question, the relevant connection is Neckar-Odenwald-Kreis → Baden-Württemberg. The German word Land in Landkreis does not make it one of the 16 federal states.

The official district-administration directory lists Mosbach for Neckar-Odenwald-Kreis. This concrete place gives you a useful second connection when studying: district name, administrative seat and federal state are different pieces of information that belong together.

The other three answers

The incorrect answers are not invented names; their locations simply do not fit this question.

A familiar place name does not replace an accurate connection.

The question does not ask for a complete list of Baden-Württemberg’s districts. You need to recognise the relevant one among four names. If names are similar, check the official directory instead of deciding only from a landscape term.

Mosbach, as the seat of the district administration, is not the state capital. That is Stuttgart. A district administration, a city administration and the state government have different responsibilities, so their seats are not interchangeable.