'In this topic' or 'on this topic'

Solution 1:

Both "in" and "on" are used with "selected topics" (ngram) and as well with "seminar" (ngram).

If the topic treats of some part of history "in" should be used because a part is always considered to be in the whole and never on it. If the topic is not historical but instead is about history, as for instance a philosophical discussion of the importance of history, then you should use "on" because this is a usual preposition in that case (with "about").

Before you hold a seminar you must decide first what it is going to be about; it is a concept similar to "discussion" in that respect; you do not say "a discussion in sth" but "a discussion on/about sth"; this can be checked here: "in" is not used after "discussion" to introduce the topic of it.

(OALD)

• discussion with somebody (about somebody/something)
  ♦ We had a discussion with them about the differences between Britain and the US.
• discussion about somebody/something A lively discussion followed about whether he should be allowed to join the club.
• discussion on something
  ♦The pair held informal discussions on a number of topics.
• discussion around something
  ♦The exhibition offers opportunities for frank discussions around sensitive issues.
• discussion of something
  ♦ We want to encourage public discussion of mental health issues.

It is therefore more consistent, better, to say "a seminar on sth"; however "in" is not wrong as quite a number of cases exist where "in" is used to introduce the subject of the seminar (this can be checked here). Notice in the ngram that "on" is used more often than "in"; as "in" is used also to introduce the place and the time where is conducted a seminar, the curve for "in" as introducing the subject only is lower than the curve actually shown for this preposition.