meaning of "dropping in on portions"

[Lam tung]

English Language Tests, Intermediate level

ESL/EFL Test #156 "Idioms with the phrasal verb drop", question 6

Visitors attending the annual Open House have the option of either participating in information sessions for their entire duration or ......... portions of the seminars based on their schedule and interest.

(a) dropping out
(b) dropping in
(c) dropping behind
(d) dropping in on

English Language Tests, Intermediate level

ESL/EFL Test #156 "Idioms with the phrasal verb drop", answer 6

Visitors attending the annual Open House have the option of either participating in information sessions for their entire duration or dropping in on portions of the seminars based on their schedule and interest.

Correct answer: (d) dropping in on

Your answer was: correct
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Hi!
i don't understand the meaning of this sentence.
please explain it to me.
Thanks.
Lam tung

[Mister Micawber]

.
Visitors can choose between participating in everything and briefly visiting parts of the seminars.
.
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Native English teacher at Mister Micawber's