What category this question belongs to?

I found this interesting problem and couldn’t able to solve it.
Can you help me identify the category of this problem? so that I can practice similar questions.
so far my understanding is, we have to calculate the numbers of groups.
For example, Suppose pair of friends are
A B, D B, R G, T G
so there are two groups
first one is A B D, second is R T G.
so the answer is 1 because we can convert this into one group by just making one friendship between the first group and second group.
below is the problem statement.

ICL21D

Read about connected components in a graph and disjoint sets union.

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Thanks.

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