Right Answer:
me caí y me partí la pierna
(I fell and broke my leg)
Usually sadness comes when things do not go well or as we expected; and one of the manifestations of a sad person is to cry.
In this sentence, the subject is expressing his sadness through crying.
Now, what was the cause of the crying?
Between the two options:
a.me caí y me partí la pierna
(I fell and broke my leg)
b. el partido estuvo buenísimo
(The game was great)
It is obvious that the answer is option a.
Therefore, the whole sentence is:
Me puse triste y lloré mucho porque me caí y me partí la pierna
(I got sad and I cried a lot because I fell and broke my leg)