Question

what is one of the biggest problems with excessive irrigation?
soil salinization (increased saltiness of the soil)
plants drying out too quickly

Explanation:

Excessive irrigation leads to waterlogging, and as water evaporates, salts from the water (and underlying soil layers) accumulate in the topsoil, causing soil salinization. Plants drying out too quickly is not a result of excessive irrigation (excessive water would keep soil moist, not dry plants quickly). So soil salinization is the main problem here.

Answer:

soil salinization (increased saltiness of the soil)