The incorrect statement regarding the rule to find the oxidation number among the following is-

1. The oxidation number of hydrogen is always +1.

2. The algebraic sum of all the oxidation numbers carried by elements in a compound is zero.

3. An element in its free or uncombined state has an oxidation number of zero.

4. Generally, in all its compounds, the oxidation number of fluorine is -1.

Hint: Flourine is the most electronegative element thus it never show positive oxidation state
Oxidation number of hydrogen is always +1 is a wrong rule since, it is +1 in hydrogen halide, -1 in hydrides and zero in H2 molecule.
All the other three statements (b), (c) and (d) are correct.