What is the colour of the nano gold particles?
Explanation
Option D is correct because the provided text explicitly states that the colour of gold nanoparticles is size-dependent. The text mentions that nanoparticles smaller than 30 nm are 'ruby red', those up to 100 nm are 'pink', and larger ones appear 'darker'. Since the colour changes based on the size of the particle, it is described as variable. Option C (Red) is only partially correct for particles smaller than 30 nm, whereas the question asks about nano gold particles in general.
Solution Steps
Step 1: Read the context regarding the colour of gold nanoparticles.
Step 2: Note the specific colours mentioned: 'ruby red' for sizes smaller than 30 nm, 'pink' for sizes up to 100 nm, and 'darker' for larger sizes.
Step 3: Observe that the text demonstrates a 'change in one of the physical characteristics' (colour) based on size.
Step 4: Conclude that since the colour varies with size, the correct answer is 'Variable'.