Given repetition is not allowed.
$\therefore$ First place can be filled in 10 ways.
Second place can be filled in 9 ways.
Third place can be filled in 8 ways
Fourth place can be filled in 7 ways
$\therefore$ Total number of ways =$10\times 9\times 8\times 7$
$\Rightarrow 5040$ ways
Total number of outcomes n(S)=5040
Only one sequence will be right which will open the lock
No of favorable cases n(E)=1
Hence (A) is the correct answer.