Which of the following would indicate a failing implant?

An implant is considered to be failing when there are signs of instability or inflammation that may compromise its longevity. Pain upon percussion is a significant indicator of potential issues with the implant. It suggests that there might be an underlying problem, such as bone loss or infection around the implant. Pain during percussion can indicate that the implant is not properly integrated into the bone or that there are complications that need to be addressed.

Mobility of 75 micrometers is quite minimal and does not typically indicate failure, as micro-mobility can occur without being a sign of failure unless it becomes more pronounced. A 5mm pocket interproximally without exudates suggests potential peri-implant health issues but isn't definitive for failure, particularly in the absence of symptoms like exudation or significant inflammation. Gingival recession of the machined or polished collar can occur for various reasons and does not inherently indicate that the implant is failing, as stability and integration might still be intact despite aesthetic or soft tissue concerns.

Thus, pain upon percussion is a clear, actionable symptom that signals potential implant failure, making it the correct answer in this scenario.