Correct option is D
Given: The series is: 168, 191, 218, 249, 284, ?
Concept: (+23, +27, +31, +35, +39) Pattern follows and its double difference is 4.
Solution:
168+23=191
191+27=218
218+31=249
249+35=284
284+39=323
So, 323 is the missing number.