How does the magnitude of the lever arm will change if you decrease the angle?

it decreases.
it stays the same.
it increases.

It decreases.

Explanation:

Lever arm is defined as the "perpendicular distance that exists from the rotation axis to the line of action of the force". Being its magnitude related to the application of the force that produces a torque.  

To understand it better:  

The lever arm is an effective distance to apply a force with respect to a certain point, and this distance serves as an effective applied force amplification factor.

In this sense, Torque is defined as:

Where:

is the distance from the applied force to the fulcrum

is the applied force

is the angle between and

It is important to note, is maximum when . Then, assuming and remains the same, if decreases decreases, as well.

Hence:

If the angle decreases, the magnitude of the lever arm will decrease.

So, the greater the lever arm, the greater the torque and the lesser the force we have to apply.

wavelegth as in spectrum red oygbiv

conservation of mass

mass before something happens = mass afterwards