a) The data frame is a key data structure in statistics and in R
b) R has an internal implementation of data frames that is likely the one you will use most often
c) There are packages on CRAN that implement data frames via things like relational databases that allow you to operate on very very large data frames
d) All of the mentioned

Answer: d
Explanation: The basic structure of a data frame is that there is one observation per row and each column represents a variable, a measure, feature, or characteristic of that observation.