What made knights obsolete?

Answer 1

If you mean soldiers in full plate armor, the gun was probably the most important invention in making them obsolete. Their armor worked pretty well against arrows but not bullets.
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The above is certainly conventional understanding and taught in many schools. But actually, the fully armored knight was already obsolescent when the guns appeared on the field. The massed English archers of the Battles of Crecy and Agincourt destroyed formations of charging knights at long range with little damage to the English themselves. So the invention of massed archers with English longbows is one that has to be considered.

The only advantage in using musketeers of the time instead of archers with longbows was that the musketeers could be trained in a week, but training archers was a process that took years. Many kings did not want to have large numbers of archers, because they represented a threat to the king in times of peasant unrest.

Answer 2

Many things made them obsolete, but probably the one major item was gunpowder. The armour didn't provide much protection against the new weapon. To be a knight was also very expensive. The cost of armour, in today's terms, was the price of a 5 bedroom house in a nice neighborhood. They rode special horses that were also very expensive and the horses were trained to act in a particular fashion on the field of battle. War changed and men had to change with the new methods.