The condition of the earth’s atmosphere at a given place and time is known as
a. climate
b. precipitation
c. temperature
d. weather

D weather.  

It could not be the climate because that is the condition of an area over a long period.  An example of climate is how Antarctica is always extremely cold.

It would not be Precipitation as well because that is the stuff that falls to the ground like snow, rain, and hail. 

And finally though it may seem like temperature is the right answer, it is not because temperature only deals with the degree to which something is cold or hot. 

You can assure D is your answer by analyzing the excerpt of 'at a given place and time' in simpler terms this is like how during the october months for northeast USA it is substantially colder at nighttime.

The northeast is your place and at night is your time.  

The answer is
d) weather

Migration people get up and go cause they feel it is best for them