An isotope is a variant (in weight) of a chemical element. Since the number of protons in an atom of a specific element is fixed the only way to change mass is to change the number of neutrons. The atomic number for boron is 5, thus whichever answer we choose must contain 5 protons. To total a mass of 11 amu 6 neutrons must be present eliminating choice 2. In order for this isotope to be neutral the number of protons and eletrons must match one another leaving only choice 5 remaining as the correct answer.
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