How do planes descend for landing?
If the pilot reduces the airspeed while he increases the rate of descent, you will not notice any change in pitch attitude when the aircraft descends. With jets you should be able to hear a change in engine pitch when thrust is reduced. Turboprops run their engines at constant speed, and a reduction in power here means a reduction in propeller pitch. This causes the propeller noise to change, but this change is harder to discern. With a little practice you should be able to hear the slight drop in propeller noise which signals the begin of the descent, though.

Gliders control their sink rate by flying in rising or sinking air, and additionally by flying slow (and near their minimum sink rate) or fast. To descend quickly, they can open speed brakes and/or sideslip. My favorite for a fast descent is to spin the aircraft, but airliners do not use this option.

In order to produce the necessary lift to keep the aircraft in the air, the pilot picks a combination of speed and angle of attack at which lift equals weight. Other combinations would result in an accelerating vertical speed and are only used in transitions from one flight state to the next.

To arrive at the aircraft's pitch attitude, you need to add the flight path angle to this angle of attack. If the plane descends with a flight path angle of -3° and the pilot reduces speed such that he needs to increase the angle of attack by the same 3° to produce the same lift, the resulting pitch angle of the aircraft would stay the same.

Normally, the correct answer to your question is 2. Reduction of engine power. But the other answers are also correct: If the speed is held constant, the attitude has to be reduced by the desired flight path angle change. Note that this is achieved by reducing thrust alone. If the pilot wishes to descend really fast, he might move to answer 3. Combination of both as in this answer.