Option 1: Cadmium is a toxic heavy metal that can be released into the environment through various human activities such as industrial emissions and waste incineration. It is not commonly associated with automobile emissions. Therefore, it is unlikely to be a major pollutant in the air of a city with a large number of automobiles.

Option 2: Lead is a highly toxic metal that used to be a common component in gasoline. However, leaded gasoline has been phased out in many countries due to its harmful effects on human health and the environment. Despite this, older vehicles may still emit lead if they are fueled with leaded gasoline. Therefore, lead can be a significant pollutant in the air of a city with a large number of automobiles, especially if older vehicles are still in use.

Option 3: Chromium is a metal that can be released into the air through industrial processes such as the burning of fossil fuels and waste incineration. However, it is not specifically associated with automobile emissions. Therefore, chromium is less likely to be a major pollutant in the air of a city with a large number of automobiles.

Option 4: Nickel is a metal that can be released into the air through processes such as industrial emissions and combustion of fossil fuels. However, it is