Final answer:
Barium-135 undergoes positron emission to become Cesium-135. This decay process involves a proton transforming into a neutron and emitting a positron, resulting in a decrease in the atomic number by one.
Step-by-step explanation:
When Barium-135 undergoes positron emission, it emits a positron, and the atomic number decreases by one. This occurs because a proton within the nucleus is converted to a neutron and a positron; the positron is then emitted from the nucleus. Since barium has an atomic number of 56, the isotope formed will have an atomic number of 55. The element with atomic number 55 is cesium (Cs), so the isotope produced is Cesium-135 (Cs-135). Therefore, the correct answer to the question is 'B) Cesium-135'.