**Core Concept**
The acrosome reaction is a critical event in the process of fertilization, involving the release of enzymes from the acrosome cap of spermatozoa. It allows the sperm to penetrate the zona pellucida of the oocyte and facilitate fertilization.

**Why the Correct Answer is Right**
The acrosome reaction is triggered by the binding of the sperm to the zona pellucida, leading to a cascade of intracellular signaling events. This includes the activation of phospholipase C (PLC), which hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to generate inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 then triggers the release of calcium ions (Ca2+) from the endoplasmic reticulum, which in turn activates protein kinase C (PKC) and other downstream effectors. The resulting increase in intracellular calcium and protein kinase activity leads to the exocytosis of the acrosome cap, releasing enzymes such as hyaluronidase and acrosin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the acrosome reaction does not occur in the presence of high levels of estrogen, which actually inhibit sperm capacitation and the acrosome reaction.
**Option B:** This option is incorrect because the acrosome reaction is not a result of the binding of the sperm to the oolemma, but rather to the zona pellucida.
**Option C:** This option is incorrect because the acrosome reaction is not a result of the sperm's motility, but rather a response to specific ligands on the zona pellucida.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the acrosome reaction is a calcium-dependent process, and abnormalities in calcium signaling can lead to impaired sperm function and infertility.

**Correct Answer: D. Sperm binding to zona pellucida**