A.Just about any kind. Financial securities tend to have special regimes that have been set up to facilitate clearance and settlements in repo markets.
B. A general description of the collateral ("approximately 3 tons of rice stored at X company's warehouses in Managua, nicaragua" for example) is filed in a public filing system. In the case of vehicles, the exact vehicle can be described, since it has a VIN number. The borrower must agree to the filing and it's text.
C. In most places where secured transactions work well, the legal framework provides for the lender to seize the described collateral in the event that the borrower defaults. The borrower presents a statement to a judge, justice of the peace or similar stating that the borrower has defaulted and the lender backs up the trucks to the warehouse and loads the rice. In some places a bailiff is used to seize the collateral.