Chapter 1 - Functions and Their Graphs - Section 1.4 Library of Functions; Piecewise-defined Functions - 1.4 Assess Your Understanding - Page 85: 13

Identity function (B).

We can see that the graph is a straight line, which matches with the identity function $y=x$ (a line with a slope of $1$). In other words, the $x$ and $y$ values are equal along the graph at each point. So, the matching graph is $(B)$.