The country who's economy can be described as developing is most likely to engage in which kind of economic activity

The correct answer is - secondary economic activity.

The developing countries are trying to boost and develop their economies by developing the industry. They build lots of industrial facilities, factories, and instead of selling the raw materials to the more developed countries, they start to use the raw materials for their own purpose and manufacture them into products ready for the market. This production can include anything from creating steel from the iron ore, make juices from the fruits and vegetables, use the cotton for making clothes etc., so all in all making final products that directly end up on the market, thus making a bigger profit.