Composers, like ravel, that wrote music experimenting with timbre were considered to be

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Those composers were considered to be Impressionists.

Explanation:

In context of Ravel's works experimenting with timbre is related with impressionism since the composer used sounds of different musical instruments in a solo compositions. This means, for example, that the sounding of quitar and piano are different. However, Ravel used such techiques that guitar sounding was used in a piano solo works. Another example is the sounds in nature. See as a reference Jeux d'eau ("Play of Water"). There is no such technical characteristic of the piano to produce the sounds of water, but Ravel reflected his impressions from its sounds and look in his piano work.

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your answer would be d.)

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