A certain sum was invested in a high-interest bond for which

I'd go with C (both combined suffice).

1) Gives us the rate of interest. Knowing x is an integer, and the value doubles, and rate of return is between 39 and 45, its easy to figure out that x = 2 months, and rate is 41%. But this doesn't tell is the principle or final amount. Insufficient.

2. For one month more the total amount would have been $2744. Since both time and rate and principle is unknown this doesn't lead us anywhere.
For example, at 100% rate of return, the principal would have been 2744/(2^x+1). But since x is not know, nor is the rate we can't do a thing.

Combine them, and you know the rate and the time and the final amount for 3 months. Easy to get the principle.

Hence (C).

Both statements sufficient to answer.
I'll go with 'C'

what is OA?

OA is C. :-D