Option-1: Zirconium is wrong because Monazite is not an ore of this element. Zircon ore (ZrSiO4) is the primary source of zirconium.

Option-2: Thorium is correct as Monazite is a primarily reddish-brown phosphate mineral that contains rare-earth elements. Its main primary elements are thorium, uranium, and lanthanides, making it a good source of thorium.

Option-3: Titanium is incorrect. Although titanium is present in many minerals like Ilmenite and Rutile, it is not significantly present in Monazite.

Option-4: Iron is incorrect. While iron ores typically include minerals such as magnetite and hematite, Monazite is not a recognized source for iron.

So, the correct answer is Thorium, Option-2. Monazite is majorly used as a source of thorium, lanthanides, and uranium.