4) as a volume of air expands due to heating, the density of this air will a) increase b) decrease c) remain…

Brief Explanations:

Density is defined as mass per unit volume ($\rho=\frac{m}{V}$). When air is heated and expands, its volume $V$ increases while the mass $m$ remains constant (assuming no air is added or removed). According to the density - volume relationship, when the denominator ($V$) in the density formula increases and the numerator ($m$) is constant, the density $\rho$ decreases.

Answer:

B. decrease