a. Four major causes for the increase in public debt:

1. Revenue shortfalls: The Nigerian government often faces revenue shortfalls due to fluctuations in oil prices, which is the main source of government revenue. This leads to borrowing to fund budget deficits and maintain government operations.
2. Increased capital expenditures: The need for large investments in infrastructure projects, such as roads, railways, and power plants, has forced the government to borrow heavily to finance these capital-intensive projects.
3. Devaluation of the Naira: The depreciation of the Naira against foreign currencies increases the burden of external debt. As the value of the Naira falls, the cost of repaying foreign-denominated loans increases, leading to a higher public debt level.
4. Debt servicing requirements: The requirement to service existing debts, including both principal and interest payments, has forced the government to borrow even more, creating a debt spiral. High interest rates also exacerbate the problem, as more borrowing leads to higher interest obligations.

b. Three major benefits and two adverse effects of public debt:
Benefits:

1. Infrastructure development: Borrowing allows the government to invest in critical infrastructure projects that support economic growth, such as roads, power plants, and hospitals, which would otherwise be unaffordable within the constraints of the national budget.
2. Economic stimulation: Public debt can stimulate economic growth during periods of recession or economic downturn. By borrowing and investing in the economy, the government can create jobs, boost demand, and increase productivity.
3. Addressing fiscal deficits: Public debt helps the government manage its fiscal deficits when tax revenues are insufficient to cover public expenditures. Borrowing enables the government to finance essential services without drastically cutting spending.

Adverse Effects:

1. Debt servicing burden: High levels of public debt increase the amount the government must allocate to service debt (repaying interest and principal), leaving fewer resources for essential services such as healthcare, education, and security. This can strain the budget and lead to underfunding of critical sectors.
2. Potential for economic instability: Excessive borrowing can lead to unsustainable debt levels, which may cause loss of investor confidence and higher borrowing costs. This could result in inflation, currency devaluation, and overall economic instability if the debt cannot be effectively managed.