Here's why:

* Conglomerate is a sedimentary rock formed from cemented gravel. Gravel is defined as rock fragments larger than 2 millimeters in diameter.

* Sandstone is made of sand-sized particles (between 0.0625 and 2 millimeters).

* Siltstone is made of silt-sized particles (between 0.0039 and 0.0625 millimeters).

* Claystone is made of clay-sized particles (smaller than 0.0039 millimeters).

Therefore, conglomerate has the largest particles among these common sedimentary rocks.