The correct form is b, "replaced." The verb phrase "will be sent" is the first member of the alternative "or" pair, "will be sent," and therefore the second member of the pair would, in its full form, be "will be replaced."
It will be sent to us for repair or will be replaced at no cost
Since we do not normally repeat auxiliaries, we have
It will be sent to us for repair or replaced at no cost
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