St. petersburg:

a) has always been the country's most important black sea port.
b) was always a competitor for moscow, but did not become the capital of russia until the 1917 revolution.
c) lies at the head of the sea of okhotsk.
d) was named leningrad during soviet times.

d) Was named Leningrad during Soviet times.

Explanation:

St petersburg is easily the second most important city in Russia, and has always been the second most important city, it´s not the capital of Russia, Moscow is and has always been a competitor with the capital, during soviet times St Petersburg was called Leningrade in honor to Lenin, its located in the Baltic Sea, so the only one that is true is d) Was named Leningrad during Soviet times. and that is the correct answer.

iii. and iv. only is the right answer

answer conditional instability;