The correct answer is (b) 50 db.
The World Health Organization (WHO) has set a guideline for noise levels in cities at 50 decibels (dB). This level is considered to be safe for human health, and exposure to noise levels above this can have a number of negative effects, including hearing loss, sleep disturbance, and stress.
The WHO recommends that cities take steps to reduce noise levels, such as by planting trees and installing noise barriers. They also recommend that people take steps to protect themselves from noise, such as by wearing earplugs or using noise-canceling headphones.
The other options are incorrect because they are either too low or too high. A noise level of 45 dB is considered to be very quiet, and a noise level of 60 dB is considered to be moderately loud. Exposure to noise levels above 60 dB for prolonged periods of time can have a number of negative effects on human health.