What is the primary cause of a 'spongy' or soft feeling when pulling a motorcycle's front hydraulic disc brake lever, and how is it corrected?

1. Air bubbles are trapped inside the brake fluid lines; it is corrected by bleeding the brake system.
2. The brake pads are worn down to their steel backing plates; they must be replaced.
3. The brake disc rotor is slightly dusty; it must be wiped with engine oil.
4. The brake fluid reservoir is overfilled above the upper limit line.