My mather___too much money___her pet every month

Pay money for
Spend money in doing sth
Spend money on sth/sb
Cost sb money to do sth/Sth cost sb money

so, it is obvious, C

c
payfor 表示金钱

spend on 花费……在……上

cost 不能用于以人为主语的句子 只能用于以物为主语的句子

for example the pencil cost me ten yuan .

My mather_spends_too much money_on_her pet every month
C.spends, on

选c
spend sth on sth

C