Answer:
The following which represents the graph becoming more narrow is:
B. kf(x)
Explanation:
We are given a parent function f(x) as:

and k=3
Now, we know that the transformation of the type:
f(x)+k
shifts the graph of the function k units upward while there is no change in the shape of graph.
Similarly the transformation f(x+k) and f(x-k) shifts the graph of the original function k units to the left and k units to the right but odes not change the shape of the function.
Hence, the answer is:
B. kf(x)
Since, the transformation is a vertical compression and the graph becomes more narrow.