Positron emission, which is also sometimes referred to as Beta Decay, is a time of radioactive decay in which a positron is emitted. This positron is like any other electron only that it is positively charged. This means that after the emission of a positron, the atomic number is decreased by 1 which the mass number remains unchanged. For the positron emission of Co-56 with mass number of 56 and the atomic number of 27, the new atomic number is 26 and the same mass number. The atom with atomic number of 26 is iron (Fe). Thus, the answer is Fe-56.