**Core Concept**
The development of motor skills and language in children follows a predictable pattern. Sitting with support, transferring objects from one hand to another, and speaking monosyllabic babbles are significant milestones in a child's cognitive and physical development.

**Why the Correct Answer is Right**
At 6 months, infants typically begin to sit with support, which is a significant achievement in gross motor development. This milestone is often accompanied by the development of fine motor skills, such as transferring objects from one hand to another. Additionally, language development accelerates around this age, with children starting to produce monosyllabic babbles, which are the first steps towards articulating words. This language development is closely linked to the maturation of the brain's language centers, particularly the left hemisphere.

**Why Each Wrong Option is Incorrect**
**Option A:** 3 months is too early for these milestones. At 3 months, infants are primarily focusing on developing their gross motor skills, such as lifting their head while on their stomach and reaching for toys.

**Option C:** 9 months is also too early for some of these milestones. While infants may start to sit without support and transfer objects, monosyllabic babbles typically begin around 6 months.

**Option D:** 12 months is too late for monosyllabic babbles. By 12 months, children are usually saying a few words and starting to combine them to form simple sentences.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that developmental milestones can vary significantly in children, and some may need more time to achieve certain skills. However, persistent delays in reaching these milestones can be indicative of underlying developmental or neurological issues.

**✓ Correct Answer: B. 6 months.**