If a part of the lens is covered, it's focal length
(a) Increases 
(b) remains same
(c) decreases 
(d) none of the above 

Dear Student,
If the part of the lens is covered there will be no change in the focal length of the lens, it will remain same. 
Hence, option (b) is correct.
Regards

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Dear student,
When half of the convex lens is covered, say with black paper, the rays of light occurs and image will be formed as how the normal lens receives, which means that the focal length remains the same, but, only the brightness or intensity of the light will be diminished due to the covering of black paper.
Hence, If half part of a convex lens is covered, the focal length does not change, but the intensity of the image decreases.
Hope it will help you
Regards ,
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