Correct Answer - Option 4 : The data given in both statements I and II together are necessary.

**As per the statement**:

I. **30 students have a mobile phone in the classroom**

From this statement, we **cannot** conclude the total number of students in the class.

As the total number of students is unknown, this statement is alone not sufficient to answer how many students in the class do not have a mobile phone.

II. **There are a total of 50 students in the classroom**

From this statement, we **can** conclude the total number of students in the class.

Thus** using both the statements** we can say:

I) Total number of students in the classroom = 50

ii) 30 students have a mobile phone in the classroom.

**Number of students who don't have a mobile phone = 50 - 30 = 20**

∴ As it can be seen that both statement are required to obtain the number of students who don't have mobile phones.

Hence, the correct answer is "The data given in both statements I and II together are necessary".