Let's evaluate each given statement:
a. C125 is a subgraph of C173:
In graph theory, a subgraph is a graph formed from a subset of the vertices and edges of a given graph. Since a Cyclic graph with 125 vertices is smaller than a Cyclic graph with 173 vertices, it's possible for the C125 graph to form a subset of C173. Therefore, statement a is true.
b. Ky is a subgraph of K173:
In this case, we do not have a specific number for 'y'. However, in graph theory, a complete graph, K, composed of 'y' vertices can be a subgraph of another complete graph with more or equal vertices. So if for instance, 'y' was 150 or any number less than or equal to 173, Ky would indeed be a subgraph of K173. Without a specific 'y', we can't definitively say this is true.
c. C173 is a subgraph of C173:
This statement is true based on basic graph theory principles. Any graph can be a subgraph of itself because it can form a subset with all its vertices and edges within itself. Therefore, a Cyclic graph with 173 vertices (C173) is a subgraph of itself. Hence, statement c is true.
d. W125 is a subgraph of K173:
A Wheel graph (W125) with 125 vertices can be a subgraph of a Complete graph (K173) with 173 vertices. This is because 125 vertices can easily form a subset within the 173 vertices. Therefore, statement d is also true.
Given these explanations, the true statements are a, c, and d.