Which of the following best characterizes a narrowband FM (NBFM) signal?

Correct option is C

​$$\textbf{Explanation} \\[6pt]\text{For a narrowband FM (NBFM) signal:} \\[6pt]\bullet \ \text{The modulation index is defined as} \\[6pt]\beta = \frac{\Delta f}{f_m} \\[8pt]\text{where} \\[4pt]\Delta f = \text{ maximum frequency deviation} \\[2pt]f_m = \text{ maximum modulating frequency} \\[10pt]\textbf{Condition for NBFM} \\[6pt]\beta < 1 \ \Rightarrow \ \Delta f \ll f_m \\[10pt]\text{This means:} \\[6pt]\bullet \ \text{Frequency deviation is small} \\[2pt]\bullet \ \text{Bandwidth is narrow} \\[2pt]\bullet \ \text{Only the first pair of sidebands is significant}$$​​