The answer is: Parliamentarism is exercised by the prime minister.
Details:
Parliamentarism is a system of government in which the legislature (parliament) provides political support (direct or indirect support) to the executive branch. Therefore, the executive power needs the power of parliament to be formed and also to govern.
The advantage of the parliamentary system over the presidentialist is that the former is more flexible. In the case of a political crisis, for example, the prime minister can be swapped quickly and the parliament can be fired. In the case of presidentialism, the president fulfills his mandate to the end, even if there are political crises.