The correct option is C couldn't
Here, 'couldn't' is correct because it is used to show that something is or might not be possible. Here, due to rain, she 'couldn't' play tennis.
'Won't is incorrect because it is used to show that somebody/something was not willing or refused to do something.
'Always' is incorrect because it implies at all times; on every occasion.
'Shouldn't' is incorrect because it is used to show what is right, appropriate, etc., especially when criticizing somebody’s actions.