Of the possible answers you give here, the only correct one is D.

During the 1950s, US automakers frequently changed the style of cars.  They would come out with a new model of each car each year.  There was no real reason for this other than to try to create a constant market for new cars.  It was considered important by many Americans that they should have a style of car that was not too far out of date.  This marketing strategy worked, and car sales (and therefore employment) remained high.

Of course, the rest of the economy needed to be strong for people to afford these cars and so there are other reasons for why employment remained high.  However, of the answers you give, D is the only correct answer.