What property of platinum makes it suitable as an external electrical contact in cells?

Platinum is an excellent choice for external electrical contacts in electrochemical cells primarily because it is both conductive and inert. As a noble metal, platinum has high electrical conductivity, which allows efficient transfer of electrons within the electrochemical cell. This property is crucial for the overall efficiency and performance of electrochemical processes, such as those found in batteries and fuel cells.

In addition to its conductivity, platinum's inertness means that it does not readily react with the environment or the chemicals used in the electrochemical cell. This stability is essential because it ensures that the electrical contact remains effective over time without being degraded or consumed by reactions that could occur during operation. Unlike other materials that might corrode or form unwanted compounds, platinum maintains its properties, thus extending the lifespan and reliability of the cell.

The other options present properties that are not favorable for external electrical contacts. For instance, being a poor conductor would hinder electron flow, while reactivity with other substances could lead to degradation of the contact. Lastly, low cost is not associated with platinum, as it is a precious metal, and thus not a practical characteristic for an effective electrical contact in high-performance applications.