Final answer:
Portugal's economy diversified significantly after joining the European Union, shifting from traditional products to a strong service sector, including tourism and technology.
Step-by-step explanation:
After Portugal joined the European Union, its economy experienced significant changes. The correct answer to how Portugal's economy changed after joining the EU is A. It diversified from traditional products to a large service sector. This diversification involved a shift away from the reliance on traditional industries and agricultural products to the development of a robust service industry, including tourism, technology, and financial services. Moreover, with access to the larger European market and structural funds from the EU for economic development, Portugal saw improvements in infrastructure and an influx of foreign investment, contributing to economic growth and modernization. While Portugal still faces economic challenges, it has not seen a decrease in trade or a diminishing tourism sector; therefore, other options B, C, and D do not accurately describe the changes in Portugal's economy post-EU accession.