Which of these is a single replacement reaction?

A single-replacement reaction (also called a single-displacement reaction) is one in which a pure element and a compound react chemically so that the products include another pure element and a different compound via exchanging of two species.

In other words, it is where an element reacts with a compound and replaces one component of the compound:

Let's examine each option:

• (A)

Option (A) is the reaction

#"Ba(NO"_3")"_2 + "NiSO"_4 rarr "BaSO"_4 + "Ni(NO"_3")"_2#

This is a typical example of a double-replacement reaction (or double-displacement reaction), in which two substances exchange their cations to form two new substances:

Since this is obviously not a single-replacement reaction, we can eliminate (A).

• (B)

Option (B) is the reaction

#2"Fe" + 3"Br"_2 rarr 2"FeBr"_3#

This is what's called a synthesis (or combination) reaction, where two reactants combine to form only one product:

This is not a single-replacement reaction, so we can also eliminate (B).

• (C)

Option (C) is the reaction

#"CaCl"_2 + "Na"_2"CO"_3 rarr "CaCO"_3 + 2"NaCl"#

This, like answer (A), is a double-replacement reaction, so we eliminate (C).

This leaves us with one final option:

• (D)

Option (D) is the reaction

#color(red)("Fe") + color(blue)("Cu")"SO"_4 rarr color(blue)("Cu") + color(red)("Fe")"SO"_4#

We can see from the color code that this is a single replacement reaction, conforming to the first image above, so option (D) is correct.