Which of the following affects how well a wave can penetrate obstacles?

Wavelength plays a crucial role in determining how well a wave can penetrate obstacles. As a general principle in wave propagation, longer wavelengths tend to be better at diffracting around obstacles and penetrating through materials compared to shorter wavelengths. This is particularly relevant in radio communications, where lower frequency signals (which correspond to longer wavelengths) can better penetrate through buildings, trees, and other physical barriers.

In contrast, higher frequency signals (shorter wavelengths) are more susceptible to absorption and reflection when encountering such obstacles, leading to poorer penetration. Therefore, the properties defined by the wavelength of a signal significantly impact its ability to effectively propagate through various environments.

The other options, while they contribute to overall communication effectiveness, do not directly influence penetration through obstacles in the same manner as wavelength does. Transmitter design relates more to signal strength and modulation efficiency, receiver sensitivity affects the ability to detect weak signals, and wave amplitude is more associated with signal strength and quality rather than penetration capabilities.