In the Kreb`s cycle, also known as the citric acid cycle, a series of chemical reactions occur to produce energy in the form of ATP. The correct chronological order in which the listed products are formed in the Kreb`s cycle is option 3: 2, 4, 3, 1.

First, oxalo-acetic acid (option 2) is formed. This compound is a key starting material in the Kreb`s cycle and is involved in the regeneration of citric acid. Next, a-ketoglutaric acid (option 4) is formed through a series of reactions. This compound is an important intermediate in the cycle. Then, succinic acid (option 3) is produced. Finally, fumaric acid (option 1) is formed as the last product in the Kreb`s cycle.

It is important to note that the Kreb`s cycle is a cycle, so the products are continuously regenerated to keep the cycle going. The order in which the products are listed represents a snapshot of a single round of the cycle.