I think the answer is E.

49 people read at least one of 3 kinds of magazines last year. How many people read only one kind of magazine?

(1) 3 persons read all 3 kinds of magazines
This just tells is the region of triple overlap. Nothing else is known about any of the other regions.
Insufficient.
(2) 5 persons read only 2 kinds of magazines
This just tells us all of the regions of double overlap...nothing about any other region

Combo:
We still don't know anything else about the other regions.
_________________

If each of the 49 people read at least one of the 3 kinds of magazines, then the following should be true:
49=the number of people who read only 1 kind of magazines + the number of people who read only 2 kind of magazines + the number of people who read only 3 kind of magazines.
the number of people who read only 1 kind of magazine=49-the number of people who read only 2 kind of magazines - the number of people who read only 3 kind of magazines.

(1) 3 persons read all 3 kinds of magazines - Insufficient. We don't know the number of people who read only 2 kind of magazines.
(2) 5 persons read only 2 kinds of magazines - Insufficient. We don't know the number of people who read only 3 kind of magazines.

Both statements taken together, we can find the number of people who read only 1 kind of magazines. C)