The correct option is D Each
Distributive pronouns refer to persons or things one at a time. A distributive pronoun is always singular and should be followed by a singular noun and verb. The given sentence refers to 'little groups' and then goes on to refer to the groups in singular 'his'. The verb used is also singular 'had'. This leads us to the distributive pronoun "each" in option D which sees the members of the little groups as individuals. Thus, this is the correct answer.
Option A "some" is an indefinite pronoun and can be used to describe either singular or plural nouns. However, it should be followed by the third person plural determiner 'their".
Option B "all" is not used with the singular "his".
Option C "every" is incorrect as is used as a way of seeing a group as a series of members. However, the sentence sees them as individuals.
Option E "other"is used to refer to a person or thing that is different or distinct from one already mentioned or known about. This is incorrect in this context.
Therefore, option D is the correct answer.