To understand the behavior of a single-phase induction motor at synchronous speed when only the main winding is excited, we need to evaluate the characteristics of forward and backward rotating fields:

1. Single-Phase Induction Motor Characteristics: A single-phase induction motor has two components of magnetic fields: forward rotating field and backward rotating field. The two fields are generally equal when the motor is at standstill.
2. Operation at Synchronous Speed: At synchronous speed, the slip \(s\) is 0 because slip is defined as \(s = \frac{N_s - N_r}{N_s}\), where \(N_s\) is synchronous speed and \(N_r\) is rotor speed. At synchronous speed, the rotor speed equals the synchronous speed.
3. Rotor Current: The rotor current depends on slip. Since the slip is zero at synchronous speed, the rotor current due to the backward rotating field would be insignificant or zero, and hence, the backward field itself diminishes.
4. Comparison of Fields: As a result, at synchronous speed, the influence of the backward field decreases, making the forward rotating field stronger than the backward field.
5. Conclusion: Therefore, in the given scenario, the forward rotating field is more than the backward rotating field, which explains the behavior of a single-phase induction motor at synchronous speed.

Based on this understanding, the correct answer is: Forward rotating field is more than the backward rotating field.