To determine which pair of rational numbers is equivalent, we need to simplify each fraction and check if they have the same value. Let's simplify each option:

a) 24/40 and 35/50:
To simplify 24/40, we can divide both the numerator and denominator by their greatest common divisor, which is 8. So, 24/40 simplifies to 3/5.
To simplify 35/50, we can divide both the numerator and denominator by their greatest common divisor, which is 5. So, 35/50 simplifies to 7/10.
Since 3/5 and 7/10 are not equal, option a) does not form a pair of equivalent rational numbers.

b) -25/35 and 55/-77:
To simplify -25/35, we can divide both the numerator and denominator by their greatest common divisor, which is 5. So, -25/35 simplifies to -5/7.
To simplify 55/-77, we can divide both the numerator and denominator by their greatest common divisor, which is 11. So, 55/-77 simplifies to -5/7.
Since -5/7 and -5/7 are equal, option b) forms a pair of equivalent rational numbers.

c) -8/15 and -24/48:
To simplify -8/15, we can divide both the numerator and denominator by their greatest common divisor, which is 1. So, -8/15 remains the same.
To simplify -24/48, we can divide both the numerator and denominator by their greatest common divisor, which is 24. So, -24/48 simplifies to -1/2.
Since -8/15 and -1/2 are not equal, option c) does not form a pair of equivalent rational numbers.

d) 9/72 and -3/21:
To simplify 9/72, we can divide both the numerator and denominator by their greatest common divisor, which is 9. So, 9/72 simplifies to 1/8.
To simplify -3/21, we can divide both the numerator and denominator by their greatest common divisor, which is 3. So, -3/21 simplifies to -1/7.
Since 1/8 and -1/7 are not equal, option d) does not form a pair of equivalent rational numbers.

Therefore, the correct answer is option b) -25/35 and 55/-77, as they both simplify to -5/7, forming a pair of equivalent rational numbers.

-25/35=-5/7
-55/77=5/-7
-5/7 = 5/-7(-5*-7=35 and 5*7=35)