The correct answer is: B. Magnet
A tape recorder should not be kept near a magnet because it can erase the magnetic tape. The tape recorder uses a magnetic field to record sound onto the tape. If the tape is exposed to a strong magnet, the magnetic field will be distorted and the sound will be erased.
The other options are not as likely to cause problems for a tape recorder. A clock does not produce a strong enough magnetic field to erase the tape. An electrical switchboard may produce a strong magnetic field, but it is unlikely to be close enough to the tape recorder to cause problems. A radio may produce a strong magnetic field, but it is usually turned off when not in use, so it is not likely to be a problem.
In conclusion, a tape recorder should not be kept near a magnet because it can erase the magnetic tape. The other options are not as likely to cause problems for a tape recorder.