**Core Concept**
Growth monitoring in children is a crucial aspect of pediatric care, involving the assessment of physical growth and development to identify potential health issues. The best indicator of growth monitoring should be a reliable, easily measurable, and sensitive parameter that reflects overall health and development.

**Why the Correct Answer is Right**
Weight for age is a widely accepted and effective indicator of growth monitoring in children. It takes into account the child's current weight in relation to their age, providing a comprehensive picture of their growth trajectory. This parameter is particularly useful in identifying underweight or overweight children, who may be at risk for various health problems.

**Why Each Wrong Option is Incorrect**
**Option A:** Height for age is an important parameter, but it is less sensitive than weight for age in detecting growth issues, especially in children under 2 years old. It primarily reflects chronic malnutrition or growth hormone deficiency.
**Option B:** Body Mass Index (BMI) for age is a useful indicator, but it is not as widely accepted as weight for age due to its complexity and the need for accurate height measurements. It is more relevant for identifying obesity or underweight in older children.
**Option C:** Mid-upper arm circumference (MUAC) is a simple and effective indicator of acute malnutrition, particularly in emergency settings or low-resource areas. However, it is not as comprehensive as weight for age in assessing overall growth and development.

**Clinical Pearl / High-Yield Fact**
In children, growth monitoring should be a regular part of health check-ups, with weight for age being the primary indicator. This allows for early identification and intervention of growth-related issues, improving health outcomes and preventing long-term complications.

**Correct Answer: D. Weight for age.**