Two identical ideal diodes are connected to an ammeter and a d.c source (1 volt) as shown. In which one of the following circuits, ammeter will not show any deflection?
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In a series loop, a single reverse-biased diode is enough to completely block the current. Always look for the configuration where a diode's blocking line ($n$-side) directly faces the incoming positive current flow from the battery source without any alternative parallel paths.
Step 1: Recall the diode rule. An ideal diode conducts only when forward biased, that is when its anode (the $p$ side) is at a higher potential than its cathode (the $n$ side). Reverse biased, it is an open switch. Step 2: Think series versus parallel. In a single series loop a current can flow only if every diode in the loop is forward biased. One reverse biased diode breaks the whole loop. Step 3: Scan the four circuits. We look for the arrangement where the only available path is blocked by a reverse biased diode. Step 4: Identify the blocked loop. In circuit (a) the diodes are placed so that the conducting path meets a diode whose $n$ side faces the higher potential terminal of the $1\,\text{V}$ source. That diode is reverse biased. Step 5: Trace the consequence. With this diode acting as an open switch and no parallel bypass, no charge can circulate, so the ammeter current is zero. Step 6: Contrast the rest. The other circuits offer at least one forward biased route, so they carry current and the meter deflects. Hence (a), option (B). \[ \boxed{\text{circuit (a)}} \]
