When Philip analyzed unknown substance x, he found that it contained only one kind of atom what is substance x

Substance X is a (B) Element.
Explanation:

A compound consists of either many identical atoms or more than 1 type of atom. They can be broken down or converted into a simpler one.
An element is a substance which contains only one kind of atom. It cannot be broken down further or converted into simpler substance by any chemical means.

Heterogeneous mixture is a non uniform mixture of different atoms without any chemical bonding in between them.
Homogeneous mixture is a uniform mixture of different atoms without any chemical bonding in between them.
From the definitions it is clear that substance X is (B) Element.