First place goes to "Opportunity Cost"

From What do economic scholars consider powerful economic knowledge of importance for people in their private and public lives? Implications for teaching and learning economics in social studies by Niclas Modig.

Based on Young’s notion of powerful knowledge, acquiring disciplinary knowledge emerging from an economic epistemic community is expected to make an important difference for people when dealing with economic issues in their daily lives. In this regard, this article’s author asked Swedish scholars of economics at higher education institutions what they considered to be the most important economic concepts that people would need to acquire and understand. The article provides knowledge about the case of Sweden as a contribution to existing research. Although the article does not suggest a list of key concepts, the results clearly bear close similarities to the economic concepts proposed within the framework of threshold concepts. These concepts is one important resource to be considered when deciding on what economic content students should have access to in school enabling them to face economic issues in their private and public lives.

From the findings.

As shown in Figure 3, opportunity cost is the largest category when the individual (74) and the citizen perspectives (60) are merged, with a total of 134 occurrences, followed by interest, with 51 occurrences (33 from the individual perspective and 18 from the citizen perspective), and marginal concepts, with 49 occurrences (33 from the individual perspective and 16 from the citizen perspective).

A comparison shows that 10 categories (opportunity cost, interest, marginal concepts, demand and supply, inflation, scarcity, tax, market, GDP/growth and unemployment) are amongst the top 15 in each perspective. Five categories (benefit, risk, sunk cost, budget constraints and price) amongst the top 15 in the individual perspective do not qualify for the top 15 when both perspectives are merged. Five categories (external effects, incentives, efficiency, equilibrium and welfare) are amongst the top 15 in the citizen perspective but not when both perspectives are merged. Figure 3 presents the 20 categories from both perspectives, although some categories do not make it to the top 15 when the perspectives are merged. An example is the welfare concept in 14th place in the citizen perspective, but it falls to the bottom in the individual perspective, with only one occurrence.

Opportunity cost is closely related to trade-offs.  There is a cost to all proposals and there are constraints (time, money and talent) which force you to make trade-off decisions among those proposals.  

Something which all politicians and mainstream media deny as does much of academia.  Everyone else in the real world knows these concepts intimately.