What math subject would Intro to Quantitative Business Skills fall under?

All of them would be helpful. But note the degree of difficulty between each one is quite diverse. Anyway assuming you have had basic math, alegbra, geometry/trig, etc., I found the following useful:

Advanced Statistics (take this after Stat)

Linear Algebra (take this after Calc Single Variable)

Multivariable Calculus (take this after Linear Algebra)

Differential Equations (take this after Multi Calc)

Discrete Mathematics (take this last but only if intending to take computers)

Calculus Single Variable (take this before Multi Calc)

Statistics (take this before the Advanced version)

While these are important skills to learn for quantitative analysis the course contents vary significantly from school to school. For example in Advanced Stat make sure you get multivariate regression analysis and the design and analysis of experiments.