In the 1950s, los angeles became famous for its smog. one part of the problem was

The problem was an increase in the number of cars.

Many people will tell you that Los Angeles is the city of the automobile. They famous LA highways and roads are featured in many forms of art be that movies or novels. The car is a symbol of Los Angeles. But that also created many problems such as high traffic, which results in long standstills and increase in pollution.

Your answer is d)magnesium