**Core Concept**
The ureter's entry into the bladder is a crucial anatomical aspect, involving the **ureterovesical junction**. This junction is significant for preventing **vesicoureteral reflux**, where urine flows back from the bladder into the ureters. The **trigone of the bladder** is a key landmark in this context.

**Why the Correct Answer is Right**
The correct answer involves the ureter's entry at the **lateral angle of the trigone**, which is a precise anatomical location. This entry point is crucial for the **ureteral orifice** to function correctly, allowing urine to flow from the ureters into the bladder while preventing reflux. The **intramural portion of the ureter** acts as a natural **anti-reflux mechanism**.

**Why Each Wrong Option is Incorrect**
**Option A:** Entry at the medial angle of the trigone is incorrect because this is where the urethra begins, not the ureter's entry point.
**Option D:** The ureter's course is not straight as it enters the bladder; it is rather oblique, contributing to its anti-reflux mechanism.

**Clinical Pearl / High-Yield Fact**
Remembering that the ureters enter at the **lateral angles of the trigone** is crucial for understanding urinary tract anatomy and preventing complications like vesicoureteral reflux. This knowledge is essential for urological procedures and surgeries.

**Correct Answer:** b. At lateral angle of trigone