Genetically modified plants have been useful in:

The correct option is: Making crops tolerant to stresses.

Genetically modified (GM) plants are plants whose DNA has been modified using genetic engineering techniques. One of the primary goals of creating GM crops is to enhance their resilience and productivity under challenging environmental conditions.

Let us analyze the options to understand why the selected option is correct:
1. Making crops tolerant to stresses: Abiotic stresses such as cold, drought, salt, and heat significantly limit crop productivity worldwide. Genetic modification allows scientists to introduce specific genes (e.g., genes regulating osmotic balance or stress-responsive proteins) into plants, making them more resilient to these environmental pressures. Therefore, this is a highly useful application of genetically modified plants.
2. Increasing post-harvest losses: This is incorrect. GM plants are actually designed to reduce post-harvest losses (for example, by delaying ripening or improving shelf life, like the Flavr Savr tomato).
3. Decreasing crop yield: This is incorrect. GM crops are engineered to increase overall yield by reducing losses from pests, diseases, and environmental stress.
4. Decreasing efficiency of mineral usage by plants: This is incorrect. GM plants are often modified to increase the efficiency of mineral usage, preventing early exhaustion of soil fertility and reducing the need for chemical fertilizers.

Thus, genetically modified plants have been remarkably useful in making crops tolerant to abiotic stresses.