The maturation phase of enamel formation is a critical stage in the development of tooth enamel. During this phase, the primary focus is on the mineralization of the enamel matrix and the removal of enamel proteins that were initially deposited during the secretory phase. This process ensures that the enamel becomes highly mineralized, hard, and durable.

To elaborate:

- In the initial secretory phase, ameloblasts secrete an organic matrix primarily composed of enamel proteins such as amelogenins.
- However, during the maturation phase, these proteins are broken down and removed, allowing for increased mineral deposition.
- This leads to a significant increase in the inorganic content (primarily hydroxyapatite crystals) of the enamel, which is essential for the hardness and resilience of mature enamel.

The other options are incorrect because:

- Option 1 ("Deposition of organic matrix and cellular proliferation") describes the secretory phase, not maturation.
- Option 3 ("Vascular invasion and collagen fiber formation") is more relevant to dentinogenesis or bone formation rather than enamel maturation, as enamel is an avascular tissue and does not contain collagen.
- Option 4 ("Apposition of dentin and enamel junction") refers to the initial formation of the dentinoenamel junction (DEJ), which occurs before the maturation phase.

Therefore, the correct answer is Option 2: Mineralization and removal of enamel proteins.

Reference:
Ten Cate's Oral Histology, 9th Edition, Chapter 4: Enamel Formation / Page 85-90