When the levels of trp inside a cell are high, transcription of the trp operon stops due to formation of a caused by base pairing between regions within the leader region.
a. stop codon, 2& 3
b. trp codons, 3& 4
c. stem-loop, 2& 3
d. stem-loop, 3& 4

D. stem-loop, 3&4

Explanation:

When tryptophan levels are high, sequence 1 of leader peptide is quickly translated as it has two "trp" residues. Ribosome translates sequence 2 before sequence 3 is transcribed. This prevents the stem-loop formation between sequence 2 and 3.  

The continued transcription and translation of the complementary sequences 3 and 4 followed by the formation of a stem-loop structure by these two sequences serve as an attenuator for transcription of trp operon.