D

Step 1: Understand the concept of harmony in sound.
Tones are considered harmonious when their frequency ratios are simple whole numbers.
Step 2: Identify the frequency of Tone A, which is 240 Hz.
Step 3: Calculate the ratios for each option with respect to Tone A (240 Hz):
- Option A: 240 Hz → ratio = 240/240 = 1 (harmonious)
- Option B: 480 Hz → ratio = 480/240 = 2 (harmonious)
- Option C: 360 Hz → ratio = 360/240 = 1.5 (less harmonious but still simple)
- Option D: 450 Hz → ratio = 450/240 = 1.875 (not a simple ratio and thus least harmonious)
Conclusion: The frequency that will sound least harmonious with Tone A is 450 Hz.
Therefore, the correct answer is D.