応訴 is read as おうそ (ouso) and means countersuit.
おうそ
ouso
おうよう
applied (e.g. physics, linguistics, research)
おうじる
to respond
おうえん
cheering
おうたい
dealing with (people, customers, complaints, etc.)
おうぼ
application
おうとう
reply
おうせつ
reception (e.g. of visitors)
おうずる
to satisfy
こたえる
おうぶん
according to one's abilities
おうきゅう
emergency
おうしょう
responding to a call-up for military service