Which is the graph of f(x)=100(0.7)?

I think you meant:  "Which is the graph of f(x)=100(0.7)^x?"  Unfortunately, you have not shared the four possible answer choices.

Graph   f(x)=100(0.7)^x:

Recognize that this is a decaying exponential function, since the common ratio, 0.7, is between 0 and 1.  Each time x increases,s f(x) decreases.

Table of x, y values:

 x     y
 0    100(0.7)^0 = 100(1) = 100
 1     100(0.7)^1 = 70
 2     100(0.7)^2 = 100(0.49) = 49
 and so on.

The graph never crosses or touches the x-axis.  There is no vertical asymptote, but there is a horiz. one:  y=0.