Co is a (a) reactant (b) reactant and a product (c) product

The equation is missing. So all the options can be possible.

Explanation:

CO is termed as carbon monoxide. Mostly CO is formed by the absence of desired amounts of oxygen on reaction with carbon based molecules. So CO will be formed as the by products instead of forming as byproducts due to less oxygen presence.

So CO can form as product. Even CO can be available naturally during volcanoes burning which releases on reaction with atmosphere. Thus in this case, CO will act as reactant. So as CO can be obtained in a product and also a reactant to form . It can act as both a reactant and a product.  

In order to understand the status of CO in a chemical reaction equation, the position of CO in the chemical equation should be noted. It is known that the left side of chemical reaction equations contains the reactants for the reaction and the right side of the chemical reaction equation denotes the products formed from the reactants.

So if CO is placed in the left side, then it is a reactant and if it is placed in right side, it will product and in some cases, if CO is placed in both the sides then CO is acting as a catalyst i.e., reactant and product in that reaction.

adding a(n) to an electric circuit would increase the magnetic field around a wire;

b. solenoid

a solenoid is a type of electromagnet whose purpose is to generate a controlled magnetic field.

if the purpose of the solenoid is instead to impede changes in the electric current, a solenoid can be more specifically classified as an inductor rather than an electromagnet.