"is being organized": This is correct because "an excursion" is a singular subject, and "is being organized" is the present continuous passive form that agrees with the singular subject.
"are being organized": This is incorrect because "are" is the plural form and does not agree with the singular subject "an excursion."
"is been organized": This is incorrect because "is been" is not a valid construction in English.
"are been organized": This is also incorrect because "are been" is not a valid construction and does not agree with the singular subject.
