The correct answers are The soup is too salty because I added too much soy sauce. and I added too much soy sauce, so the soup is too salty.

Because introduces the reason or cause (adding too much soy sauce).

So introduces the result or effect (the soup becoming too salty).

The incorrect options mix up the cause and the effect, creating a funny but illogical situation where the chef adds soy sauce as a result of the soup already being salty!