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Explanation: The first one is given in Diccionario politécnico de las lenguas española e inglsa (1998)
The second and third choices are given in Urrutia's Spanish-English Technical Dictionary for "idler gear"
The last one is given in Collazo's Spanish-English technical dictionary
You have to translate the whole phase - "idler gear" . "Idler", out of context, and still in a technical sense, has at least 15 different translations (Diccionario politécnico de las lenguas española e ingles - segunda edición Díaz de Santos, 1998)