Explanation
The cause of pollution of underground water is excessive use of (D) All of the above: insecticides, pesticides, and chemical fertilizers.
These substances, when used in excessive quantities or improperly, can leach into the soil and eventually contaminate groundwater. Overuse or improper application of insecticides, pesticides, and chemical fertilizers can lead to the accumulation of harmful chemicals in the soil, which can then infiltrate into the groundwater system and cause pollution. It is important to use these substances judiciously and follow proper guidelines to prevent groundwater contamination.