The correct answer is option 2: 118.

In this question, we are asked about the heaviest element known up until January 2015, specifically its atomic number. Atomic number refers to the number of protons in an atom`s nucleus.

Option 1, 117, is not the correct answer. It is a number close to the correct answer, but it is not the heaviest element known at the time.

Option 2, 118, is the correct answer. This is the atomic number of oganesson, which was officially recognized by the International Union of Pure and Applied Chemistry (IUPAC) in 2016.

Option 3, 119, is not the correct answer. There is no known element with an atomic number of 119.

Option 4, 120, is not the correct answer. There is no known element with an atomic number of 120.

In conclusion, the heaviest element known up until January 2015 has an atomic number of 118.