A scientist studying the genetic engineering of crops and working to create GMOs would most likely focus on option a: How to create a crop with a higher yield and greater nutritional value.
This is a primary goal of many genetic engineering projects aimed at improving agricultural outputs and addressing food security. Scientists often seek to enhance yield (the amount of crop produced per unit of land) and nutritional value (the vitamins, minerals, and overall health benefits of the crops) to better meet the needs of growing populations and improve overall health.
While options b, c, and d are also relevant aspects of crop improvement, they do not specifically align with the overarching goals of maximizing productivity and nutritional benefits, which are central to many GMO development projects.