Take the task of building 4-digit positive integers and break it into stages.

Stage 1: Choose a thousands digit
This can be 1,2,4,5,7,8,or 9, so we can complete stage 1 in 7 ways

Stage 2: Choose a hundreds digit
This can be 0,1,2,4,5,7,8,or 9, so we can complete stage 2 in 8 ways

Stage 3: Choose a tens digit
This can be 0,1,2,4,5,7,8,or 9, so we can complete stage 3 in 8 ways

Stage 4: Choose a units digit
This can be 0,1,2,4,5,7,8,or 9, so we can complete stage 4 in 8 ways

By the Fundamental Counting Principle (FCP) we can complete all 4 stages (and thus build a 4-digit positive integer) in (7)(8)(8)(8) ways

IMPORTANT: we don't really need to calculate the product (7)(8)(8)(8)
We can just recognize that the units digit will be 4. That is (7)(8)(8)(8) = ---4
Since answer choice B, is the only one with units digit 4, it must be correct.

Note: the FCP can be used to solve the MAJORITY of counting questions on the GRE. So, be sure to learn it.

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