The rivers in siberia tend to flow
a. north and are warmer at the source than at the mouth.
b. south and are colder at the source than at the mouth.
c. east and are about the same temperature at either end.
d. west and are frozen throughout the year.

b. south and are colder at the source than at the mouth

Explanation:

The great Siberian river like the Ob, Yenisei, and Lena all flow to the arctic ocean and thus flows in the east to west direction and has a relatively flat percentage of the landmass. The southern flow makes these rivers more warm as compared to the icy content that it gathers to form the northern sides. Within the impact of the winter season, these rover water get thawed and thus re-reach the southern ends with relatively less cold waters as they have a tendency to flow downward. When they are joined form the nearby seas.

Answerleveling;

The answer would be  b. hope this !