Answer: A. 7
Explanation:
Given : Point S lies between points R and T and RS=6n, ST=35 and RS=42.
Since , RS = 6n and RS = 42
It means
[By property of equality]
To find the value of n , we divide both sides by 6 , we get

Therefore , the value of n is 7.
Hence , the correct option is A. 7.