Sergei called Lushkoff his godson.

Explanation:
Lushkoff's godfather is Sergei, which means that Sergei holds a special relationship with Lushkoff. A godson is someone who is chosen by their godparent to be their spiritual child and is usually involved in their life in a significant way. In this case, Sergei has taken on the role of godfather to Lushkoff.

The term "godson" is often used in religious contexts, particularly in Christian traditions where a godparent is chosen to guide and support the spiritual development of a child. However, the term can also be used more broadly to describe a special relationship between two individuals, regardless of religious affiliation.

In the given options, the correct answer is option A, which states that Sergei is the one who called Lushkoff his godson. This means that Sergei has taken on the role of godfather to Lushkoff, showing his commitment to being a mentor and guide in Lushkoff's life.

It is important to note that the information provided in the question does not mention any other individuals who may have called Lushkoff his godson. Therefore, option B, which states that Olga called Lushkoff his godson, is incorrect. Option C, which states that both Sergei and Olga called Lushkoff his godson, is also incorrect. Similarly, option D, which states that none of the above called Lushkoff his godson, is also incorrect.

In conclusion, Sergei is the one who called Lushkoff his godson. This suggests that Sergei has taken on a special role in Lushkoff's life, providing guidance and support as a godfather.