The correct answer is B. in.
A pixel (px) is a unit of measurement used in computer graphics to represent a single point on a display device. It is the smallest addressable element in an all points addressable display device.
An inch (in) is a unit of length in the imperial and US customary systems of measurement. It is currently defined as 1/12 of a foot, or 2.54 centimeters.
An em is a unit of length used in typography. It is defined as the width of the capital M in the current font.
A point (pt) is a unit of length used in typography. It is defined as 1/72 of an inch.
In the context of the question, the only unit of measurement that defines a measurement in inches is in.