Which of the following represents a synthesis reaction?

ac + b → ab + c
a + b + c → abc
abc → a + b + c
ab + cd → ac + bd

The answer to your question is the second one

Explanation:

                                         AC + B → AB + C

This is not the right answer, the reaction shown here is a single replacement reaction in which one element replaces another element that is part of a compound.

                                         A + B + C → ABC

This is the right answer because in synthesis reactions two or more reactants bound to form only one product.

                                          ABC → A + B + C

This answer is incorrect because this is an example of a decomposition reaction in which one reactant forms two or more products.

                                       AB + CD → AC + BD

This is an example of double replacement reaction in which the cations and anions of the reactants are interchanged.