For a Fibonacci sequence, from third term onwards each term

I think the answer should be 4. Cannot be determined as the even 1 * 517 could also be considered.
In that case T6 and T7 would be 258 and 259, giving a different T10

Yes, Caligula is right that the answer should be 'cannot be determined', at least as the question is worded. They don't even tell us the numbers need to be integers, nor that they need to be positive. If you take the first solution above (where 18 and 29 are the sixth and seventh terms), and just make those numbers negative, you'll get another sequence that 'works', and -123 can be the tenth term. And if you allow non-integers in the sequence, there are infinitely many possibilities.

The question only has a unique answer if you know the terms must all be positive integers. Then the other factorization (1)(517) doesn't work, because that would force some of the earliest terms in the sequence to be negative. Either way, it's not a realistic GMAT question at all, so test takers shouldn't worry about it.