The correct answer is: 
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                        →    " commensalism" . 
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<u>Note</u>:  "Commensalism" refers to a symbiotic relationship {"symbiosis"}  between two (2) organisms — in which one organism benefits and the other organism is neither helped nor harmed.  
               { i.e.  " +, 0 " ;  that is:  "plus" sign for "one organism benefits" ; 
                        "zero" sign for "neutral" ; 
                     (in that the other organism is neither helped nor harmed.}. 
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In the example given in the question, the two (2) organisms are: 
  1)  the barnacles;  and:  2)  the shellfish.
        The barnacles benefits by given a place to attach for the purpose of transportation (by using the shellfish to "cling to"}.  
       The shellfish are neither helped (benefitted in anyways from the attachment from the barnacles— nor are the shellfish harmed in any way.
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Hope this answer helps you!
      Best wishes in you academic pursuits
                — and within the "Brainly" community!
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