The armature MMF (magnetomotive force) waveform of a DC machine is triangular. This means that the distribution of the magnetic field produced by the armature winding varies in a triangular pattern over time.

Explanation:

1. Armature MMF:
- The armature MMF is the magnetic field produced by the armature winding of a DC machine.
- It is responsible for generating the electromagnetic force necessary for the operation of the machine.

2. Waveform Types:
- There are several types of waveforms, including pulsating, rectangular, triangular, and sinusoidal.
- Each waveform type has a distinct shape and characteristic pattern.

3. Pulsating Waveform:
- A pulsating waveform is one in which the magnitude of the signal varies between maximum and minimum values.
- It is characterized by a series of pulses or spikes in the waveform.

4. Rectangular Waveform:
- A rectangular waveform has a constant magnitude for a certain period of time, followed by an abrupt change to another constant magnitude.
- It is characterized by sharp edges and flat tops and bottoms.

5. Sinusoidal Waveform:
- A sinusoidal waveform is a smooth, periodic oscillation that follows a sine function.
- It is characterized by a continuous, smooth curve with no sharp edges or corners.

6. Triangular Waveform:
- A triangular waveform is a waveform that rises linearly from zero to a maximum value, then falls linearly back to zero.
- It is characterized by a series of straight lines that form a triangular shape.

7. Armature MMF Waveform:
- In a DC machine, the armature MMF waveform is triangular.
- This means that the magnetic field produced by the armature winding varies in a triangular pattern over time.
- The magnitude of the magnetic field rises linearly from zero to a maximum value, then falls linearly back to zero.

In conclusion, the armature MMF waveform of a DC machine is triangular. This triangular waveform is a result of the armature winding's magnetic field varying in a triangular pattern over time.