An irrational number is a real number that can't be expressed as a ratio, therefore when written as a decimal, it doesn't terminate or repeat.
For your problem, we can take each of the options and solve to see which one else satisfies the definition...
A) x^2 = 1... sqrt of 1 = 1
B) x^2 = 2... sqrt of 2 =
1.414213....
C)x^3 =8 x= 2
D)x^2 =49 sqrt 49 = 7
Each of the answers when solved gives you a whole number except B which gives you a number that doesn't repeat and doesn't terminate.
Answer B