Awritten agreement
a. contract
b. investment
c. solvency
d. tariff

Answer : A. contract 

A written agreement is also called as a contract. A contract can be about business, employment, sales and others that need formal written agreement. This documented agreement is legally binding meaning this can be used as an evidence in the court whenever necessary. A contract must be issued individually to the involved parties in the form of copies may it be print or non-print format.However, contracts can become null and void if some legal matter violation. It is best to place your contract in a safe place for future purposes.