Bunuel way is the best approach but you can also use estimation if you are short on time. Looking at the answer choices, they are spread out far enough to estimate.
5^2 = 25 and 6^2 = 36. So estimate the sqrt29 to be 5.5.
Now 7 + 5.5 = 12.5 and 7 -5.5 = 1.5.
3^2 = 9 and 4^2 = 16. So estimate the sqrt12.5 to be 3.5 and Sqrt1.5 to be a little more than 1, or just assume 1.
Not you have (3.5 -1)^2 = 6.something
Looking at the answer choices, you can immediately cross of A (too small), D and E (too big).
Now you are left between b and C. If you notice b has sqrt 29, which you already assumed to be 5.5. Multiplying it by 2 will give you a number greater than 10. Answer C looks good because you have 14 - 4*2.something. This value will be closer to 6
Answer C