Answer
a. $Au^{3+}$.
b. $Fe^{3+}$.
c. $Pb^{4+}$.
d. $Sn^{2+}$.
Work Step by Step
You can find the answers to this question on page 185, topic 'Positive ions: Loss of electrons'.
A cation is a type of ion that loses valence electrons and has a positive charge. The charge of a cation is determined by the number of electrons lost by the element.