The speed of a DC motor is directly proportional to-

Correct option is C

​$$\bullet \text{The speed of a D.C motor is directly proportional to the back EMF and inversely proportional to the flux.} \\[10pt]N = \frac{E_b}{\phi} = \frac{V - I_a R_a}{\phi} \\[10pt]N \propto E_b$$​​