The Constitution of India divides powers between the Union Government and the State Governments through three lists: the Union List, the State List, and the Concurrent List.

The subjects mentioned in the Union List are exclusively under the legislative and administrative jurisdiction of the Union Government. The subjects mentioned in the State List are exclusively under the legislative and administrative jurisdiction of the State Governments. However, the subjects mentioned in the Concurrent List can be legislated upon and administered by both the Union Government and the State Governments simultaneously.

In the case of subjects in the Concurrent List, both the Union Government and the State Governments have the power to make laws and regulations. This means that both the Union Government and the State Governments can enact laws related to these subjects and can exercise executive authority over them.

Therefore, the correct answer to the question is option 'B' - State Government. The Constitution confers executive power of a subject in the Concurrent list to the State Government. This means that the State Governments have the power to exercise executive authority over the subjects mentioned in the Concurrent List, in addition to the Union Government.

It is important to note that while both the Union Government and the State Governments can exercise executive authority over subjects in the Concurrent List, in case of a conflict between a Union law and a State law on the same subject, the Union law prevails. This is because the Constitution gives the Union Government supremacy over matters in the Concurrent List in case of a conflict.

In conclusion, the executive power of a subject in the Concurrent List is conferred to the State Government. This means that the State Governments have the authority to exercise executive power over subjects mentioned in the Concurrent List, along with the Union Government.