Answer:
a. X is a binomial random variable with n = 50 and p = 0.04
b. Y is a binomial random variable with n = 40 and p = 0.015
Step-by-step explanation:
Binomial probability distribution
The binomial probability is the probability of exactly x successes on n repeated trials, and X can only have two outcomes.

In which
is the number of different combinations of x objects from a set of n elements, given by the following formula.

And p is the probability of X happening.
X: the number of US residents (out of 50) with blood type AB.
Blood type AB is the rarest blood type, occurring in only 4% of the population in the United States
This means that 
Y: the number of Australians (out of 40) with blood type AB.
In Australia, only 1.5% of the population has blood type AB.
This means that 
Z: the total number of individuals (out of 90) with blood type AB.
Here

Which of the following is true about the random variables X, Y, and Z?
Options a and b are true, while c is false.