## Core Concept
The question tests knowledge of radioactive isotopes and their emission types. Beta particle emitters are relevant in nuclear medicine for both therapeutic and diagnostic purposes. A pure beta particle emitter releases only beta particles (electrons or positrons) without gamma radiation.

## Why the Correct Answer is Right
Strontium-90 (²⁹⁰Sr) is a pure beta emitter. It decays into Yttrium-90, releasing beta particles without emitting gamma radiation. This characteristic makes ²⁹⁰Sr useful for therapeutic applications where gamma-free irradiation is preferred.

## Why Each Wrong Option is Incorrect
* **Option A:** ²³²Th (Thorium-232) is primarily an alpha emitter and also emits gamma radiation during its decay series, making it not a pure beta emitter.
* **Option B:** ¹³¹I (Iodine-131) emits both beta particles and gamma radiation, so it is not a pure beta emitter.
* **Option D:** ⁶⁰Co (Cobalt-60) is a gamma emitter and also releases beta particles, but it is not considered a pure beta emitter due to its significant gamma emission.

## Clinical Pearl / High-Yield Fact
A key point to remember is that pure beta emitters like ²⁹⁰Sr are used in certain medical therapies, such as treating certain types of bone cancer, because their radiation can be focused on the tumor without penetrating too far into surrounding healthy tissue.

## Correct Answer Line
**Correct Answer: C. ²⁹⁰Sr.**