Final answer:
The most effective method to study planetary atmospheres from the options is spectroscopic observations, which involve analyzing the spectrum of a star or planet to determine various atmospheric properties.
Step-by-step explanation:
To find information about planetary atmospheres, the most effective method mentioned is c) Spectroscopic observations. By analyzing the spectrum of a star or planet, scientists can learn about its temperature, chemical composition, and atmospheric pressure. The changes in a star's spectrum caused by a planet can reveal the existence of that planet and its characteristics, such as its orbit and mass. This is done using the Doppler effect in high-resolution stellar spectroscopy, which measures the star's radial velocity and any wobble caused by the gravitational pull of an orbiting planet.
Other methods mentioned, like analyzing seismic waves and measuring gravitational pull, are more suitable for studying a planet's internal structure or the effects of nearby astronomical bodies and local geology, rather than its atmosphere.