In which type of instrument the moving coil consists of a single loop of fine bronze or platinum silver wire ?

Option 4 : Duddell moving coil galvanometer

Duddell moving coil galvanometer:

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• It consists of a moving coil made of fine bronze or platinum silver wire.
• This wire passes over a small pulley at the top and is pulled tight by a spring attached to the pulley, the tension of which can be adjusted by turning a milled head attached to the
  spring.
• The loop of wire is stretched over two ivory bridge pieces, the distance between which is adjustable.
• When the moving coil vibrates due to the passage of A.C, the reflected beam from the mirror throws a band of light upon a scale provided for the purpose.
• When this galvanometer is under mechanical resonance conditions, the amplitude of vibrations are very large and consequently, a very wide band of light is observed on the
  scale.