The correct answer is D) the high price of school supplies.
Based on the reading, the difference between education in Costa Rica and Nicaragua is "the high price of school supplies."
We can find the key to answering this question in the following part of the text: "However, many parents have to withdraw their children from school because of financial difficulties and a lack of jobs. Parents find it hard to spend their low income on transportation, uniforms, school materials, and books when they have difficulty affording food."
The reason is that the economy in Central American countries such as Nicaragua, El Salvador or Honduras is so low, that most of the people live in poverty, have to take their children out of school to work in the farm fields or other activities, or in many cases, they decide to leave their countries to migrate to the United States.