German A2 Level Question

The correct form is 'diskutieren' (b) because 'wir' (we) is the subject, and the present tense form for 'wir' is 'diskutieren'. 'Diskutiert' (a) is used for 'er/sie/es' or 'ihr', 'diskutiertest' (c) and 'diskutiertet' (d) are past tense forms.