The correct option is A boqx
From 1st and 2nd statements, the common alphabet is 'O' and the common coded letter is 'b'.
From 1st, 5th, 6th, 8th, 9th, 10th and 12th statements, the common alphabet is 'R' and the common coded letter is 'w'.
From 5th and 9th statements, the common alphabets are 'RT' and the common coded letter are 'wx' respectively. So, 'x' stands for 'T'.
From 6th, 7th, 10th and 11th statements, the common alphabet is 'U' and the common coded letter is 'e'.
From 6th and 10th, the common alphabets are 'RUS' and the common coded letters are 'weo' respectively.
So, 'S' is coded as 'o'.
Again, from 3rd, 5th, 6th, 8th and 11th statements, the common alphabet is 'A' and the common coded letter is 's'. So, 'A' is coded as 's'.
From 8th and 11th statements, the common alphabets are 'AC' and the common coded letters are 'sq' respectively.
So, 'C' is coded as 'q'.
Therefore, 'COST' is coded as 'qbox' or 'boqx' in any order.
Hence, option (A) is the correct answer.