Explanation

(a)
Ionisation energy increases with decrease in atomic size and decrease in shielding effect. Ten $d$-electrons in Ga shield the nuclear charge less effectively than the s and p electrons. Hence, the outer electron is held fairly strongly by the nucleus. Consequently, ionisation energy slightly increases inspite of the increase in atomic size from Al to Ga. Hence, Al (IE=577) and Ga(IE=578) have approximately equal ionisation potential (or ionisation energy).