The fact of some fraternal twins resembling each other greatly and others looking quite dissimilar highlights an interesting and often overlooked feature of fraternal-twin pairs, namely they vary considerably on a spectrum of genetic relatedness.

a.The fact of some fraternal twins resembling each other greatly and others looking quite dissimilar highlights an interesting and often overlooked feature of fraternal-twin pairs, namely they vary considerably

b.That some fraternal twins resemble each other greatly while others look quite dissimilar highlights an interesting and often overlooked feature of fraternal-twin pairs, namely that they vary considerably

c.With some fraternal twins resembling each other greatly and others looking quite dissimilar, it highlights an interesting and often overlooked feature of fraternal-twin pairs, namely considerable variation

d.With some fraternal twins resembling each other greatly and others looking quite dissimilar, it is a fact that highlights an interesting and often overlooked feature of fraternal-twin pairs, namely a considerable variation

e.Because some fraternal twins resemble each other greatly and others look quite dissimilar, this fact highlights an interesting and often overlooked feature of fraternal-twin pairs, namely they vary considerably

The issues:

Namely:

When we use "Namely", we should be careful that "namely +NOUN/NOUN Phrase/Noun clause" is correct.

BUT, Namely + Clause is not correct. Whatever comes after namely MUST work as NOUN.

Example:

I have three friends namely X, Y, and Z.

--> Look at the options A and E. In both the options "namely + clause" has been used. Eliminate them.

With + COMMA + NOUN

With X, Y---> It means that Y has X.

Example:

With a good GMAT score, Perfect has gone to Yale.
-->Perfect has the GMAT score.

Look at the options C and D. They have used "with ......, it", which changes the intended meaning of the sentence and makes the sentence awkward.

Answer is B