Because hydrogen bonds hold the two strands of a double-stranded DNA molecule together, the strands can be separated without breaking any covalent bonds. Every unique DNA molecule "melts" at a different temperature. In this context, Tm (melting temperature) is the point at which two strands separate, or become denatured. Order the DNA sequences listen below according to relative melting temperatures, from lowest Tm (easiest to denature) to highest Tm. Assume that they all begin as a stable double-stranded DNA molecules (complement not shown)
A. 5'- GGCGCACC- 3'
B. 5'- TATTGTCT-3'
C. 5'- GACTCCTG-3'
D. 5'-CTAACTGG-3'

A. (lowest Tm) B, D, C, A (highest Tm)
B. (lowest Tm) A, C, D, B (highest Tm)
C. (lowest Tm) C, D, A, B (highest Tm)
D. (lowest Tm) A, B, C, D (highest Tm)

We, know that, Guanine makes triple bond with Cytosine (G≡C) and Adenine makes double bond with thymine (A=T). Thus, to denature the bonds, highest Tm or melting temperature is required for breaking the triple bond of Guanine and cytosine than adenine and thymine.

In the above question,The 1st DNA sequence contain highest G≡C bonding than (A=T) bonding thus it will require more Tm to break the DNA.

But, the 2nd DNA sequence contain more A=T bonding than G≡C bonding and thus it require less Tm or melting temperature to denature it.