Which of the following is not essential for plants?

Out of the given options, Sodium and nickel are not essential for plants.
Sixteen elements are considered essential nutrients for plants. These are carbon (C), oxygen (O), hydrogen (H), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo) and chlorine (Cl).
Although not essential for most plants, sodium (Na(+)) can be beneficial to plants in many conditions, particularly when potassium (K(+)) is deficient.
Nickel is a component of some plant enzymes, most notably urease, which metabolizes urea nitrogen into useable ammonia within the plant. But then also it is not part of the 16 most essential plant nutrients.