The correct answer is option 3: food subsidy.

In the Budget Estimates of expenditure on major subsidies during the fiscal year 2019-20, the highest amount of expenditure was expected to be on food subsidy. The term "subsidy" refers to financial support provided by the government to reduce the cost of a particular commodity or service for the general public.

Option 1, urea subsidy, is not the correct answer as it specifically refers to a subsidy provided for urea, which is a type of fertilizer used in agriculture.

Option 2, petroleum subsidy, is also not the correct answer as it refers to a subsidy given to reduce the cost of petroleum products such as gasoline and diesel.

Option 4, fertilizer subsidy, is incorrect as it is a more general term and does not specify any particular type of fertilizer, like urea.

Therefore, the correct answer is option 3, food subsidy, as it corresponds to the highest estimated expenditure on major subsidies during the given fiscal year.