A user receives an error when installing a new email app on their phone.

When an application fails to install on a smartphone despite being supported by the OS and device type, CompTIA A+ stresses that the most common cause is insufficient storage. Mobile devices require not only space for the app itself but also additional temporary space for the installation process, cached data, and updates. If internal storage is nearly full, the app store will display installation errors—even if the OS and hardware meet requirements.

Battery health has no impact on app installation. A SIM card is unrelated to installing local applications—it only affects cellular connectivity. Processor architecture issues would have been caught during compatibility checking, but the technician already verified the device supports the application.

Therefore, after confirming compatibility, the next logical step is to check available storage space. If storage is low, deleting unused apps, clearing cached data, or transferring media to cloud storage or an SD card typically resolves the installation issue.

Thus, Option A: Storage is the correct answer.