The correct option is B BD
In △ADM and ACM,
AM=AM [Common side]
∠AMD=∠AMC [AM is perpendicular to CD]
DM=MC [AM is the bisector of CD]
Hence, △ADM and △AMC are congruent... By SAS congruency
Therefore, AD=AC [CPCT]
⇒AB+BD=AC ⇒BD=AC−AB
Thus, the segment BD is equal to AC−AB.