**Core Concept**
Tumor progression refers to the sequential acquisition of genetic alterations and phenotypic changes that enable cancer cells to become more aggressive and malignant. This process involves the accumulation of mutations that disrupt normal cellular functions, leading to uncontrolled cell growth and invasion.

**Why the Correct Answer is Right**
Tumor progression is characterized by the sequential appearance of features of increasing malignancy, such as loss of differentiation, increased proliferation, and acquisition of invasive and metastatic capabilities. This process is driven by the accumulation of genetic mutations that activate oncogenes and inactivate tumor suppressor genes. The resulting changes in gene expression and cellular behavior enable cancer cells to grow, invade surrounding tissues, and eventually metastasize to distant sites.

**Why Each Wrong Option is Incorrect**
**Option A:** Spread of cancer to distant sites is actually known as metastasis, which is a result of tumor progression but not the definition of it.
**Option B:** Rate of growth of tumors is a different concept, known as tumor kinetics, which is an important aspect of cancer biology but not directly related to tumor progression.
**Option C:** Ability of cancer cells to resemble their normal counterparts is actually known as differentiation, which is often lost in cancer cells as they undergo tumor progression.

**Clinical Pearl / High-Yield Fact**
A key feature of tumor progression is the activation of oncogenes, such as HER2 and KRAS, which drive cancer cell growth and proliferation. Understanding the molecular mechanisms underlying tumor progression is crucial for developing effective cancer therapies.

**✓ Correct Answer: D. Sequential appearnace of features of increasing malignancy.**