How does a fixed-pitch propeller changes the blade's angle of attack? Studying I have found a question that states that in a fixed-pitch propeller in flight at a given TAS, the blade's angle of attack will change if RPM increase, but I don't understand how can the blade's AOA change if the pitch of the blade is fixed.
Am I missunderstanding the concept of fixed-pitch propeller or the question is wrong?  

Question answer and explanation for source:  

Q:For a fixed-pitch propeller in flight at a given TAS, the blade angle of attack will:
A:increase if RPM increases.
E:For a fixed pitch propeller the pitch angle cannot be changed (it varies from root to tip to maintain near-constant angle of attack along the blade).

The angle of attack as in the case of a aircraft wing is defined as the angle between the chord line and relative airflow. With a propeller however the relative airflow is the resultant of the airflow due to rotation and forward speed. Change either of these values and the angle of attack will change.
The pitch of the blade is fixed.  But the angle of attack depends on how the blade moves through the air.  That motion is mostly a combination of the forward speed of the airplane and the rotational speed of the blade.

In your graphic, the blade is attached to a plane that is flying up the page.  The blade is sticking out of the page and is being pushed to the right by the turning engine.

At some combination of airplane speed (up the page) and propeller speed to the right, the air would flow exactly along the (fixed) pitch of the blade.

Now, without immediately changing the speed of the plane you increase the RPM.  The blade moves to the right more quickly.  The angle of attack it makes in the air increases.

Put your hand out a car window.  Hold it at a fixed angle relative to the ground (say 15 degrees).  Now, holding the angle constant, move your hand downward rapidly.  The pitch has not changed, but the angle of attack has.  This is the equivalent of increasing RPM.