Martin luther objected to what policy of the catholic church, which prompted him to write and post his ninety-five theses

Indulgences.

Essentially, what the Catholic Church decided to do was to convince its subjects that paying it in small fees would reduce the amount of time they would spend in Purgatory (think the place between Heaven and Hell). These fees were called indulgences, and people could pay to have their or their family members' years in Purgatory slightly shaved off. This, of course, allowed it to gain much money, as people's years in Purgatory sometimes numbered in the thousands, if not the MILLIONS. 

Martin Luther was angered at such a system, finding it deceptive and corrupt. He wrote and posted his Ninety-Five Theses in defence of the Church, but with disdain towards the indulgence system.