The blank requires a noun to follow the indefinite article 'a'. Also, a noun is required to denote what is required to steer a boat. So, helping verb 'are' and conjunction 'or' are negated.
'Oar' - a pole with a flat blade, used to row or steer a boat through the water. As per the meaning, 'oar' fits in contextually.
'Ore' - a naturally occurring solid material from which a metal or valuable mineral can be extracted profitably.
The complete sentence is: You use an oar to steer a boat.
Hence, option (b) is the answer.
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