The given function is
f(x) = 8 x
Suppose , y = f(x) is function.
A vertical stretch by a factor of k means,
y = k f(x) , General formula for vertical stretch
Now the function is vertically stretched with a factor of 2.
Resulting function = g(x) = 2 f(x)= 2 × 8 x = 16 x,
Option (c) is right choice.