Like Chris said you should probably check out the John Hull book, that explains these concepts very well in the early chapters (Ch 4 and 5 of the 10th Ed.).
According to John Hull (he uses continuously compounded rates), the price of a forward should be:
$$F_0 = (S_0+U)e^{rT}$$
where $U$ is the present value of all storage costs.
The rational being: the price of a derivative, by no arbitrage conditions, should be the price that you can't make money out of by replicating it using spot trades.
Incidentally, I don't think any of the answers you present are the right one. You should determine the present value of all storage costs:
$$ U = 2.75 \times (DF_1 + DF_2 + DF_3 + DF_4 + DF_5)$$
and then apply $F_0 = (S_0+U)e^{rT}$
But the way you use the annual rate of 6.40% doesn't seem right. How is annual rate of 6.40% expressed? Continuous compounding, discretely compounding, simple rate?
When you use it as $463.25(1 + 0.064*5/12)$ you are using it as a simple rate (or with a compounding frequency higher than 5 months). But when you use it as $2.75(1 + 0.064*5/12)^5$ you are applying it as before but compounding the result 4 times.
- If it is a continuous compounded rate, you would use it as $e^{-rT}$ for the discount factors ($DF_n$) or $e^{rT}$ to get future value.
- If it a discretely compounded rate: $\frac{1}{(1+r / m)^{m}}$ for the DF and $(1+r / m)^{m}$ for future value
- If it is a simple rate: $\frac{1}{(1+r \times n)}$ for the DF and $(1+r \times n)$ for future value
