The primary reason so many Mexican corn farmers lost their livelihoods after the implementation of NAFTA (North American Free Trade Agreement) is that Mexican farmers could not compete with government-subsidized U.S. corn.
NAFTA eliminated tariffs and trade barriers between the U.S., Canada, and Mexico, leading to an influx of inexpensive U.S. corn into the Mexican market. U.S. corn production is heavily subsidized by the U.S. government, allowing U.S. growers to sell their corn at lower prices than Mexican farmers could afford to produce it. This unequal competition resulted in many local Mexican farmers being unable to sell their corn profitably, leading to widespread job losses and a decline in rural livelihoods.
The other options, while they touch on aspects related to the agricultural sector, do not directly capture the most significant impact of NAFTA on Mexican corn farmers.