<em>Isotopes</em> are those substances which have same atomic number (Numbers of protons) and different masses (Number of proton + numbers of neutrons).
Iodine also have many isotopes like
<em>Iodine-125</em> that contain 53 protons, 72 neutrons.
<em>Iodine-128</em> that contain 53 protons, 75 neutrons.
<em>Iodine-129 </em> that contain 53 protons, 76 neutrons.
<em>Iodine-131</em> that contain 53 protons, 78 neutrons.
<em>Summary:</em>
Here the Iodine-131 is correct answer of this question.