#### Answer

a. 1
b.-1
c.$ \frac{3}{7}$
d.$\pi$

#### Work Step by Step

a. A natural number is a positive integer such as 1, 2, 3, 4, 5, etc.
b. An integer is a whole number, or a number that is not a fraction such as 1 or -1. -1 is a correct answer because it is an integer but it is not a natural number, because it is negative.
c. A rational number is a number that can be expressed as a fraction. $ \frac{3}{7}$ is a correct answer because it is a fraction, and it is not a whole number so it is not an integer.
d. An irrational number is a number that cannot be represented in a ratio of two integers. $\pi$ is an irrational number because it cannot be represented in a ratio of two integers.