Hello!
The answer is: B)

Why?
An irrational number is a number which cannot be expressed like a fraction of entire numbers because there is not possible that any fraction can exactly express that number.
Let's discard each option in order to find the irrational number:
A)

So, A, is a rational number.
B)

There is not a fraction of entire numbers that can be written to express that number since it have infinite decimals.
C)

It's an exact number, so it's a rational number.
D)

It's a rational number since it can be written as a fraction of entire numbers.
Have a nice day!