In developed countries, people are generally very busy and somewhat solvent. So, someone can start a company that produces, for example, packed-sandwiches. But, in a poor country, people are not willing to pay extra cents to buy such fancy products. They will rather prefer loaves of breads, dip them in plain water and swallow. This is because, the thing matters to them is only to satisfy hunger.
Can anyone please give me a general theory so that I can understand what types of businesses must not be undertaken in such poor countries?
Also of recent interest - an NPR article on how the poor / middle / upper class spends their money.
As for what type of business to start - that depends on your target market.