Correct Answer: 100

Solution : Given:
The total number of students = 150
Let the number of students who opted for biology = x
and the number of students who opted for chemistry = y
This means –
x + y = 150 — (i)
Now, 12 students who opted for Biology changed their preference and opted for Chemistry.
So, the new number of students who opted for biology = x – 12
and the new number of students who opted for chemistry = y + 12

The number of students in Biology is half the number of students in Chemistry –
(x – 12) = (y + 12) ÷ 2
2(x – 12) = y + 12
2x – 24 = y + 12
2x – y = 36 — (ii)
Adding the equations (i) and (ii) –
x + y + 2x – y = 150 + 36
3x = 186
x = 186 ÷ 3
x = 62
Now, put the value of x in the equation (i) –
62 + y = 150
y = 150 – 62 = 88

So, the students who opted for chemistry now are (88 + 12) = 100. Hence, the fourth option is correct.