If a scuba tank (a rigid, sealed gas cylinder) heats up, the relevant effect would be:
b. The pressure exerted by the gas in the tank would increase.
According to the ideal gas law (PV = nRT), for a rigid container where the volume (V) is constant, an increase in temperature (T) will lead to an increase in pressure (P) if the number of gas particles (n) remains constant. The density of the gas, the size of the gas particles, and the number of particles in the tank would not change significantly due to heating in a sealed tank.