Underdeveloped economies are characterized by a combination of factors that hinder their overall growth and development. These economies typically face significant challenges in terms of poverty, low productivity, and limited opportunities for economic advancement. Let's examine each option to understand why all of them denote an underdeveloped economy.

a) High level of inequalities:
- In underdeveloped economies, there is often a significant gap between the rich and the poor. This inequality is usually reflected in income disparity, lack of access to basic services, and limited opportunities for social mobility.
- The presence of high levels of inequalities indicates a failure to distribute resources and opportunities equitably, which hampers overall development.

b) Low level of capital productivity:
- Capital productivity refers to the efficiency with which capital resources are utilized in the production process. In underdeveloped economies, there is generally a low level of capital productivity.
- This low productivity can be attributed to several factors such as inadequate infrastructure, limited access to technology and modern production techniques, and a lack of skilled labor.
- As a result, these economies struggle to generate sufficient output and fail to reach their full potential, contributing to their underdeveloped status.

c) A relatively closed economy:
- Underdeveloped economies often have limited integration with the global economy and rely heavily on domestic markets.
- A relatively closed economy means limited trade, restricted foreign investment, and a lack of exposure to international markets and competition.
- This isolation can hinder economic growth as it limits access to new technologies, markets, and capital inflows that are vital for development.

By considering all the options, it becomes clear that each one contributes to the underdevelopment of an economy. High levels of inequalities, low capital productivity, and a relatively closed economy are interconnected factors that reinforce each other and impede overall growth and development.

Hence, the correct answer is option 'D' - All of the above.