C

When alloys are formed from metals, their resistivity can vary significantly compared to that of their constituent metals.
Step 1: For pure metals, the resistivity is generally lower than that of alloys due to the enhanced electron mobility in pure metals compared to their alloyed counterparts.
Step 2: This means that the resistivity of alloys can be less or greater than that of the pure metals depending on the particular mixture and the interactions between the metal atoms.
Step 3: Since there is no straightforward relationship that universally applies to all alloys and their constituent metals, the best answer is that there is no simple relation between the resistivity of the alloy and its constituent metals.
Therefore, the correct option is C.