The correct answer is "Inability to pay for housing." This is because homelessness often occurs when individuals or families cannot afford the cost of housing. It may be due to low income, unemployment, or financial difficulties. In such situations, individuals may be forced to live on the streets or in temporary shelters. Other factors such as drug or alcohol usage, lack of education, and mental illnesses can contribute to homelessness, but the primary cause is the inability to pay for housing.