D. 3 psi
Explanation:
Pressure is defined as force acting per unit area and is numerically expressed as:
![P= \frac {F}{A}](/tpl/images/0548/7262/5f027.png)
where P represent pressure, F is force and A is area where the force acts. Substituting 300 lb for force and 100 sq.in for area then pressure,
![P=\frac {300 lb}{100 in^{2}}= 3 \ psi](/tpl/images/0548/7262/c6573.png)
Therefore, from the choices given, option D, 3psi is the right choice.