において における においての - Grammar Discussion

For the study question 「学校____いじめについて、もっと真剣に話し合うべきだ」 (Standard, Formal conjunctive):

When I saw “formal conjunctive,” I thought it was referring to this (連用形) and answered におけ and got the question wrong. The correct answer was における. What does “formal conjunctive” refer to here if not 連用形?