**Core Concept**
The question is testing the student's knowledge of the histopathological features of testicular tumors. The correct answer requires an understanding of the morphological characteristics of different types of germ cell tumors, which are the most common type of testicular cancer.

**Why the Correct Answer is Right**
The most common type of testicular cancer is germ cell tumor, which can be further divided into seminoma and non-seminomatous germ cell tumors (NSGCT). The histological features of NSGCT include the presence of embryonal carcinoma, yolk sac tumor, choriocarcinoma, and teratoma. The gross and histological findings in the orchiectomy specimen would likely reveal a mixed tumor with different components, making NSGCT the most likely diagnosis. The presence of embryonal carcinoma, yolk sac tumor, and teratoma would be characteristic of NSGCT.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because seminoma is a type of germ cell tumor, but it typically presents as a homogeneous, well-circumscribed tumor with a characteristic "chalky" appearance on histology. The gross and histological findings in the orchiectomy specimen would not typically reveal a mixed tumor with different components.
**Option B:** This option is incorrect because Leydig cell tumor is a type of sex cord-stromal tumor, which is less common than germ cell tumors. The histological features of Leydig cell tumor include the presence of large, eosinophilic cells with a "hobnail" appearance, which is not characteristic of the gross and histological findings in the orchiectomy specimen.
**Option C:** This option is incorrect because Sertoli cell tumor is another type of sex cord-stromal tumor, which is rare and typically presents as a well-circumscribed tumor with a characteristic "microfollicular" appearance on histology. The gross and histological findings in the orchiectomy specimen would not typically reveal a mixed tumor with different components.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that testicular cancers can present with a variety of different histological patterns, and a definitive diagnosis can only be made after histopathological examination of the orchiectomy specimen. A high index of suspicion and careful clinical evaluation are necessary to diagnose testicular cancer.

**Correct Answer: D. Non-seminomatous germ cell tumor**