Whole sentence meaning

[Kelly]

Dear Alan:
Can you explain me what does it mean?

" I won a lot of money by getting on that horse but it was simply a case of ......... luck.

(a) start's
(b) starter's
(c) beginner's
(d) beginning's "

Why we need using the beginner's here?
Can't really understand "by getting on that horse?"

Thanks and Regards,

[Torsten]

Hi Kelly,

It's betting on that horse not getting on that horse. Please click on the link below for more information.
http://www.english-test.net/forum/ftopic1405.html

Beginner's luck is an idiom which means that a person who is inexperienced in a sport or game achieves success in a particular event.

TOEIC listening, question-response: Did they discuss the new marketing plan?