The emf of an electrochemical cell is the potential difference between the two electrodes. It can be expressed in different ways depending on whether standard oxidation or reduction potentials are used:

1. In terms of both oxidation and reduction potentials: $E_{cell} = E_{oxidation}(\text{anode}) + E_{reduction}(\text{cathode})$. This matches statement (b).
2. Since $E_{reduction} = - E_{oxidation}$, we can substitute the reduction potential of the cathode to get: $E_{cell} = E_{oxidation}(\text{anode}) - E_{oxidation}(\text{cathode})$. This matches statement (d).
3. Alternatively, in terms of reduction potentials only (IUPAC convention): $E_{cell} = E_{reduction}(\text{cathode}) - E_{reduction}(\text{anode})$.

Therefore, statements (b) and (d) correctly represent the relation for the emf of a cell.