A positively-charged particle (alpha particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is:

Explanation

Here the positively charged alpha particles are moving towards west, so the direction of current is towards north, so the force is towards north. So, we are given that :
(i) direction of current is towards west, and
(ii) direction of force is towards north.
Let us now hold the forefinger, centre finger and thumb of our left-hand at right angles to one another. Adjust the hand in such a way that our centre finger points towards west (in the direction of current) and thumb points towards north (in the direction of force). Now, if we look at our forefinger, it will be pointing upward. Since the direction of forefinger gives the direction of magnetic field, therefore, the magnetic field is in the upward direction. So, the correct answer is : (d) upward.