(i) The collection of rich people in a district is not a set, because it is not well defined.
(ii) The collection of natural numbers less than 50 is a set, because it is well defined. It includes 1, 2, 3, ... 49.
(iii) The collection of the most talented persons of India is not a set, because talent can be in any field. For instance, it can be a talent of dancing or singing. So, it is not well defined. Therefore, it is not a set.
(iv) The collection of the first ten prime integers includes 2, 3, 5, 7, 11, 13, 17, 19, 23 and 29. This is a collection of well defined elements. Therefore, it is a set.
(v) Collection of all days in a week starting with the letter 'T' includes {Tuesday, Thursday }.
It is well defined hence, it is a set .
(vi) Collection of some months in a year is not a set because here it is not defined which month we have to select .Therefore, it is not a set .
(vii) The collection of all books in a school library is a set because here it is well defined about the collection of all books of the school library.
(viii) The collection of smart boys in your class is not a set because smart can be in any sense. It is not well defined here.
Therefore, it is not a set .
(ix) Collection of multiples of 7 includes { 7,14,21,28....}. Because it is well defined, therefore, it is a set.
(x) The collection of students in a class who got a lot of marks in the first unit test is not a set because lot of marks can be any number .
It is not defined here. Therefore, it is not a set.