. . . . . . . . is the default Visual Basic data type

In Visual Basic, the default data type is variant. A variant is a data type that can hold any type of data. It's a flexible data type that can store numbers, strings, dates, and other types of values. However, while it provides flexibility, it can also lead to some performance overhead and potential type-related issues.

Option A, string, is not the default data type in Visual Basic, although strings are commonly used data types.
Option B, boolean, is not the default data type in Visual Basic. A boolean data type can only hold True or False values.
Option D, date, is not the default data type in Visual Basic. While date is a common data type, variant is the default.

Therefore, the correct option is Option C: variant.