D

Step 1: Understand the scenario: The observer is traveling in the direction of one of the sirens. As he moves towards the source of sound, the frequency and wavelength of sound waves from the siren traveling towards him will change due to the Doppler effect.

Step 2: Apply the Doppler Effect: The frequency of the sound heard by the observer increases as he travels towards the source because the relative velocity between the observer and the source changes. This phenomenon is referred to as the Doppler shift.

Step 3: Analyze the situation with two sirens: Since both sirens emit sounds of the same frequency and the observer is moving towards one siren, the sound from the siren that is approached will be perceived with a higher frequency.

Step 4: Determine the interference: The sound waves from the two sirens will overlap in the space where the observer is present. Since they have the same frequency, and assuming they are in phase, constructive interference occurs when the sound waves from both sources add together to create an intensified sound.

Step 5: Conclusion: While moving towards the one siren, the observer will encounter an increase in sound intensity due to the constructive interference of the waves from both sirens perceived at that position.

Therefore, the correct answer is Option D: Will listen intensive sound due to constructive interference.