Which of the following is true about single-replacement reactions
they are restricted to metals
they involve a single product
two reactants produce two products
any metal replaced any other metal

Two reactants produce two products

Explanation:

In single replacement reaction, an ionic specie is replaced by another specie according to the following general scheme

Z + XY → ZY + X

Here Z can be a metal or a non-metal. So, this removes the first given option. Also, It can be seen from the general reaction that the product is not single deducting the second option. The last option states that, "any metal replaced any other metal", this is not necessarily true. For the replacement of metal the other metal should be more reactive.

So, the only true statement about single-replacement reaction is, "two reactants produce two products"