Answer:
A). I would tell the truth if I'd known it.
Explanation:
The conditional mood of the verb is defined as the verb form that expresses a condition or hypothetical situation under which a consequential action would take place. The first sentence i.e. 'I would tell the truth if I'd known it' displays the conditional mood as it shows the condition('if I'd known the truth') which led to the outcome('I would tell it'). Thus, option A is the correct answer.
