In a group of 120 students, 80 students are from the Science stream and the rest are from the Commerce stream. It is known that 70 students support Mumbai Indians in the Indian Premier League; all the other students support Chennai Super Kings. The number of Science students who are supporters of Mumbai Indians is

Total students are 120, out of which 80 students are from the Science stream. So, 120-80=40 students must be from commerce stream.

Let us tabulate the data:

$$x$$ denotes number of students from science stream who are MI supporters.

$$80-x$$ denotes number of students from science stream who are CSK supporters.

$$x-30$$ denotes number of students from commerce stream who are CSK supporters.

$$70-x$$ denotes number of students from commerce stream who are MI supporters.

We are asked to find the value of x.

The only constraints on x are:

i.) $$x-30\ge\ 0$$, or, $$x\ge\ 30$$

ii.) $$70-x\ge\ 0$$, or, $$x\le70$$

So, overall, $$30\le\ x\le70$$

Option A (30 or more) satisfies this condition.

So, it is the correct choice.