In the real business cycle model, this best explains an increase in real GDP above the full-employment level?

a positive technology shock

Explanation:

Technology is part of the capital factor of production, and it is the most important way of increasing the quality of capital. An increase in the quality of capital means that given a same amount of capital (e. g one machine), labor will be able to produce a larger output (an increase in productivity). Technological improvements are usually achieved through research and development.

Technological improvements also affect the land factor since they can reduce the use of natural resources and wastes, although the largest impact is usually made on labor productivity.

When a country's productivity increases, real GDP might increase above full employment level because less workers are needed to produce larger outputs.