Thinking at the margin means deciding about a.) increasing or decreasing technical know-how b.) investing with borrowed money c.) adding or subtracting one additional unit of some resource d.) maximizing goods and services

The correct answer is C.

When economic decisions are analyzed, this is typically done "at the margin". Therefore, the consequences of adding or substrating an aditional unit of a certain variable are analzyed and, depending on whether the total benefits have been increased or decreased by such adition/substraction, that action will be undertaken in practice or not.

For example, the marginal cost of producing an extra unit is calculated and, if it does not exceed the revenue earned by selling that extra unit, then the firm will decide to produce that aditional item. If the marginal cost exceeded the marginal revenue, such item would not be produced.

abraham lincoln  (february 12, 1809  – april 15, 1865) was an american statesman, politician, and lawyer who served as the 16th  president of the united states  from 1861 until  his assassination  in april 1865. lincoln led the nation through the  american civil war, its bloodiest war and its greatest moral, constitutional, and political crisis.[2][3]  he preserved the  union,  abolished slavery, strengthened the  federal government, and modernized  the u.s. economy.

i would say your answer is a)

sorry if im wrong