Explanation:

- Transverse frequency: This term does not describe the difference between two close frequencies. It typically refers to the frequency of transverse waves, which oscillate perpendicular to the direction of wave travel.

- Option 2: Beat frequency: This is the correct answer. Beat frequency is the term used for the difference between the frequencies of two waves that are close in frequency. This results in a phenomenon where the resultant wave's amplitude varies at the beat frequency.

- Normal frequency: This term doesn’t pertain to the concept of differences between frequencies.

- Angular frequency: Refers to the rate of rotation or oscillation in radians per unit time and is not related to the difference between two close frequencies.