Which of the following are true with regard to igneous rocks?
I. They are formed inside the earth, under great pressure
II. They do not occur in layer
III. They are mostly crystalline in nature
IV. They contain fossils

I. This statement is incorrect. Igneous rocks are formed from the solidification of molten material (magma or lava) from within the Earth's crust. While they are formed at high temperatures, they do not necessarily form under great pressure.
II. This statement is correct. Igneous rocks do not occur in layers like sedimentary rocks. They form when molten material cools and solidifies, often resulting in a more uniform composition and structure.
III. This statement is correct. Igneous rocks are mostly crystalline in nature because they form from the solidification of molten material, allowing mineral crystals to grow and develop.
IV. This statement is incorrect. Igneous rocks do not typically contain fossils. Fossils are more commonly found in sedimentary rocks where the preservation of organic materials occurs.

Therefore, the correct answer is Option C, which means only Statements II and III are true with regard to igneous rocks.