The correct answer is option 3, India. Infant mortality rate refers to the number of deaths of infants (under the age of one) per 1000 live births. In this question, we are asked to identify the country with the highest infant mortality rate among the given options.

Option 1, Sri Lanka, is not the correct answer as Sri Lanka has made significant progress in reducing its infant mortality rate in recent years. It currently has a relatively low infant mortality rate compared to other countries.

Option 2, China, is not the correct answer as China has also made significant improvements in reducing its infant mortality rate. It has implemented various healthcare reforms and interventions, leading to a decline in infant mortality.

Option 4, Vietnam, is not the correct answer as Vietnam has also made progress in reducing its infant mortality rate. It has implemented various healthcare programs and initiatives that have positively impacted the health and well-being of infants.

Therefore, the correct answer is option 3, India. India has a relatively high infant mortality rate compared to the other countries mentioned. Factors contributing to India`s high infant mortality rate include limited access to healthcare services, poor sanitation, malnutrition, and inadequate prenatal and postnatal care.