Final answer:
The installation of a free-enterprise system deeply influenced England's economic growth, especially during the historical period of industrialization, which provided the country with the essential factors of production, including land, labor, capital, and entrepreneurship.
Step-by-step explanation:
Among the options provided for the cause of an economic boom in England in contemporary times, the installation of a free-enterprise system is most aligned with the historical economic growth discussed. While the British rock invasion (A) and the signing of the Good Friday Agreement (C) had cultural and political impacts, respectively, neither specifically sparked an economic boom. The election of Margaret Thatcher (B) did bring about significant economic policies, but the foundational expansion and prosperity began with the historical development of Great Britain's leadership in industrialization, adoption of free trade principles, and the creation of a reliable banking system which provided the necessary capital and infrastructure for economic growth.