Answer:
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.