Correct option is D
Explanation-
All four core histones — H2A, H2B, H3, and H4 — have known variants that serve specialized roles in chromatin structure and function.
So, the correct answer is option d - Histone variants have been reported for H3, H4, H2A, and H2B.
Incorrect options-
Option a: Histone variants have been reported for H3 and H4 but not for H2A and H2B.
H2A has well-characterized variants, such as:
H2A.Z: Involved in transcriptional regulation.
H2A.X: Marked during DNA damage response.
macroH2A: Linked to gene silencing.
H2B has fewer, but still significant variants, especially in testes.
This option incorrectly excludes H2A and H2B, making it false.
Option b: Histone variants have been reported for H3, H4, and H2A but not for H2B.
This correctly includes H3, H4, and H2A.
But wrongly excludes H2B, which does have variants:
Option c: Histone variants have been reported for H3, H4, and H2B but not for H2A.
H2A has the most diverse and functionally significant variants. Saying variants haven’t been reported for H2A is factually wrong.
So this is clearly incorrect.
