Total number of seminars = $n$

The organizers initially accounted for 18 attendees for each seminar.

Total number of attendees = 18n

But space limitations allowed only 15 attendees per seminar. The rearrangement resulted in 4 seminars that were together attended by at least 93 attendees.

Which means $(n-4)$ seminars had 15 attendees each.

The final equation can be formed as follows -

(Total number of attendees in $(n-4)$ seminars) + Total number of attendees in remaining 4 seminars = Total number of attendees in $n$ seminars where space limitations were not taken into consideration

$(n-4)*15 + 93 = n*18$

$15n - 60 + 93 = 18n$

$18n - 15n = 93 - 60$

$3n = 33$

$n = \textbf{11}$

Hence, Answer is B