This is an math/way answer, and I don't have an explanation to the problem, but I hope this is good enough.
Mean number of errors in each page = 0.01
Mean number of errors in 100 pages = 0.01*100=1
It is possible to use the cumulative distribution function (CMF), but the math is a little more complex, involving the gamma-function. Tables and software are available for that purpose.
Thus it is easier to evaluate with a calculator for the individual cases of k=0,1,2 and 3.
The Poisson distribution has a PMF (probability mass function)


with λ = 1
=>




=>

or
P(k<=3)=
0.9810 (to four decimal places)
Since the whole length of the rectangle is 8, and the line across isnt exactly in the middle, (but just a little bit farther than the 4 mark) ... Then the correct answer is:
D. 4 1/2 units
Well on a table of data, if the y coordinates increase a lot over a short x distance then it will be steep.
In an equation, you would put it into y = mx + b form and whatever your m is will be your slope. If m is large, it will be steep.