Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.

YZ, BCD, FGHI, ?, QRSTUV

From the left end, each cluster's letters are increased one by one. For example, the first cluster has two letters, then the second cluster has three letters. On the way, the third, fourth, and fifth clusters will have four, five, and six letters respectively.

So the letter in place of the question mark will have five letters. By this option (d) can be eliminated.

The last letter of each cluster '+2' will form the first letter of the next cluster. In each cluster, letters are in sequential form.

YZ ==> Z+2 ==> B ==> BCD

BCD ==> D+2 ==> F ==> FGHI

FGHI ==> I+2 ==> K ==> KLMNO

KLMNO ==> O+2 ==> Q ==> QRSTUV

Hence 'KLMNO' will come in place of the question mark.