c
On moving from left to right right across a period. The proton number and magnitude of nuclear charge of elements increase gradually leading to corresponding decrease in the atomic radii of the elemennt. Again, on moviniig down a group, with increase in the atomic number of element, their atomic radii gradually decrease due to addition of new shells. for isoelectronicc ions, with decrease in positive charge of cations and with increase in negative charge of anions, ionic radii increase.