A. Being less capable of maintaining thermal regulation of their bodies as - as is used to show similarity, here they want to show contrast between babies born to term and pre-mature babies therefore using as is not correct. Use of than is warranted.
B. Less capable of maintaining thermal regulation of their bodies compared to - While compared to structure is correct - compare X to Y can also be written as compare to x, y using a modifier structure, Less ...to is incorrect because less...than is a correct comparison structure.
C. Less capable of maintaining thermal regulation of their bodies than are - Correct. [Pre-mature babies, are less capable ......than babies born to term, and they must.....]
D. As they are less capable of maintaining thermal regulation of their bodies like - same as A, use of as here is pointing towards similarity between babies born to term and pre-mature babies not contrast.
E. Although they are less capable of maintaining thermal regulation of their bodies than - although is showing contrast correctly, in this case, a negative feature must be used with a positive here to justify using although. Not happening here. Although less capable but must be placed in an incubator vs less capable compared to X, Y must be placed in an incubator.