The graph below shows the half life values of parent isotopes. The bar graph shows 'Parent Isotope' labeled on the x axis and 'Half-life (in billion years)' labeled on the y axis. The first bar is labeled Uranium-238 and its height is 4. The second bar is labeled 'Thorium-235' and its height is 14, the third bar is labeled 'Potassium-40' and its height is 1.5', and the fourth bar is labeled 'Rubidium-87' and its height is 49.

Based on the graph, it can be concluded that the isotope which is most likely to be found with 6.25% of its original amount in 6 billion years is

Uranium-238
Thorium-235
Potassium-40
Rubidium-87