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I suppose it is :
       d)  both A and B.

Actually this question is from some experiment to calculate the focal length of a lens or mirror.  The answer is understood only in that context.
1 5 1
why not only b
If you want to find the focal length roughly, you can do that by A, but of you are doing an experiment using optical bench, then it is option B.
if this a question of sample paper then which option should i choose?