Which of the following best describes formatted input?

Formatted input in SAS refers to the use of SAS formats, often called informats, to read and interpret raw data correctly during data input. This process allows SAS to transform data from its raw form into a format that can be analyzed. For instance, if you have a date in a character format (like '2023-10-01'), a corresponding informat can be applied so that SAS understands it as a date value, allowing for appropriate date calculations and manipulations.

The other options might refer to different methods of handling data input, but they do not accurately capture the essence of formatted input. For example, data placed in fixed columns pertains more to traditional data input techniques where each variable has a predetermined position. Data values separated by tabs relates more to delimited input methods whereby fields are delineated by tab characters. Reading data based on user-defined structures involves creating custom structures but not specifically involving formatted input as defined through SAS informats. Thus, the utilization of formats/informats effectively characterizes formatted input in SAS programming.