What are the different types of databases and which is the most common?

There are relational, object$-$oriented, and multidimensional databases. Of these, the relational database is most common.

There are relational, object$-$oriented, and multidimensional databases. Of these, the object$-$oriented database is most common.

There are relational, object$-$oriented, and multidimensional databases. Of these, the multidimensional database is most common.

There are relational, object$-$oriented, and flat file databases. Of these, the flat file database is most common.