Which of the following metals can be used as reducing agents? 

Calcium 

The correct answer is option 1. Calcium.

The highly reactive metals such as sodium, calcium, aluminium, etc., are used as reducing agents because they can displace metals of lower reactivity from their compounds. For example, when manganese dioxide is heated with aluminium powder, the following reaction takes place –

3MnO₂(s) + 4Al(s) → 3Mn(l) + 2Al₂O3(s) + Heat