The correct answer is A. m = 2j – 3.
A simple truss is a truss that has one degree of freedom at each joint. This means that each joint can only move in one direction. A truss with more than one degree of freedom at each joint is called a complex truss.
The number of members in a truss is equal to the number of joints plus the number of external loads. The number of external loads is equal to the number of reactions at the supports.
The number of equations of equilibrium for a truss is equal to the number of degrees of freedom at each joint plus the number of external loads. This means that the number of equations of equilibrium for a simple truss is equal to the number of joints plus the number of external loads.
The number of members in a simple truss is equal to the number of joints plus the number of external loads. The number of equations of equilibrium for a simple truss is equal to the number of joints plus the number of external loads. Therefore, the number of members in a simple truss must be equal to the number of equations of equilibrium.
The number of members in a simple truss can be calculated using the following formula:
$m = 2j – 3$
where $m$ is the number of members and $j$ is the number of joints.
Option B is incorrect because it does not satisfy the condition that the number of members in a simple truss must be equal to the number of equations of equilibrium.
Option C is incorrect because it does not satisfy the condition that the number of members in a simple truss must be equal to the number of joints plus the number of external loads.
Option D is incorrect because it does not satisfy the condition that the number of members in a simple truss must be equal to the number of equations of equilibrium.