Although dozens of New York's small museums are either devoted to local history or various ethnic groups, there are many one-of-a-kind museums from Manhattan to the Bronx that are open for exploration on summer weekends.

(A) Although dozens of New York's small museums are either devoted to local history or various ethnic groups, there are
(B) Although dozens of New York's small museums are devoted to local history or various ethnic groups, requires either for the or
(C) Dozens of New York's small museums are devoted to local history or various ethnic groups, but there are requires a modifier to correctly modify this clause
(D) Dozens of New York's small museums are devoted to local history or various ethnic groups, and there are also also requires a modifier to correctly modify this clause
(E) Devoted to local history or various ethnic groups, dozens of New York's small museums and also meaning of the sentence is lost and it is incorrect

IMO D.
In A,B,C there is suggested some kind of a transition. But if you read carefully, the two prats of the sentence do not contradict each other. So there is not need for a But or Although here.
D makes more sense and avoids any unnecessary transition.

If D indeed is the OA..hope my bit has helped.

GmatKiss wrote:Although dozens of New York's small museums are either devoted to local history or various ethnic groups, there are many one-of-a-kind museums from Manhattan to the Bronx that are open for exploration on summer weekends.

(A) Although dozens of New York's small museums are either devoted to local history or various ethnic groups, there are
(B) Although dozens of New York's small museums are devoted to local history or various ethnic groups,
(C) Dozens of New York's small museums are devoted to local history or various ethnic groups, but there are
(D) Dozens of New York's small museums are devoted to local history or various ethnic groups, and there are also
(E) Devoted to local history or various ethnic groups, dozens of New York's small museums and also

Detailed explanation needed.

GmatKiss wrote:Although dozens of New York's small museums are either devoted to local history or various ethnic groups, there are many one-of-a-kind museums from Manhattan to the Bronx that are open for exploration on summer weekends.

(A) Although dozens of New York's small museums are either devoted to local history or various ethnic groups, there are
(B) Although dozens of New York's small museums are devoted to local history or various ethnic groups,
(C) Dozens of New York's small museums are devoted to local history or various ethnic groups, but there are
(D) Dozens of New York's small museums are devoted to local history or various ethnic groups, and there are also
(E) Devoted to local history or various ethnic groups, dozens of New York's small museums and also

Detailed explanation needed.

(A) Although dozens of New York's small museums are either devoted to local history or various ethnic groups, there are((what's the need for although, infact it seems incorrect))
(B) Although dozens of New York's small museums are devoted to local history or various ethnic groups,((same here))
(C) Dozens of New York's small museums are devoted to local history or various ethnic groups, but there are((again, no need of contrast))
(D) Dozens of New York's small museums are devoted to local history or various ethnic groups, and there are also((i don't see any problem with this one))
(E) Devoted to local history or various ethnic groups, dozens of New York's small museums and also((no independent clause in this complete sentence))