Aformant is

a. a mix of different harmonics.
b. a frequency between two different resonant modes.
c. a resonant mode of your vocal tract.
d. a resonant mode of your vocal cords

Our final answer is option (c) that is :

A resonant mode of your vocal tract.

Explanation:

Lets understand pitch of a sound first:

Pitch: Is the rate of vibrations produced and also the degree of lowness and highness of a tone.

Pitch formant:

When we take air  inside our vocal tract it oscillates at different pitches depending upon the cast and capacity of the opening of the vocal tract.

Formant comes from the vocal tract and also at a resonance of our vocal tract.

So basically the vocal tract play a vital role for the formant.

Final answer is : A formant is a resonant mode of our vocal tract.