2. Luckily, the poor boy had enough money

网友回答

with which he could buy a train ticket是with+which开头的定语从句,修饰现行词money,which引导定语从句,在从句中作介词with的宾语,代指money.
buy a train ticket with (the money)是buy sth with (...money)句型,with 常接money,指“花钱”,“用钱”.
======以下答案可供参考======
供参考答案1:
with the money用这些钱，这个定语从句先行词是money,故选C
供参考答案2:
with the money是固定搭配