B and C are absurd; if a series converges, it must have a sum, but if a series diverges, it cannot have a sum.
Now, notice that

That is, we can write the sum more compactly as

The series is geometric with common ratio

, so the series converges (and thereby has a sum), so the answer is D.
The answer is choice A.
We're told that the left and right walls of the cube (LMN and PQR) are parallel planes. Any line contained in one of those planes will not meet another line contained in another plane. With choice A, it's possible to have the front and back walls be non-parallel and still meet the initial conditions. If this is the case, then OS won't be paralle to NR. Similarly, LP won't be parallel to MQ.
4g+3(-3+4g)=1-g
Distribute the 3(-3+4g)
4g-9+12g=1-g
Add 4g and 12g
16g-9=1-1g
Add 1g and 16g (variables always have an invisible 1 in front)
17g-9=1
Add 9 and 1
17g=10
Divide both sides by 17 and the answer is g=10/17