Select the grammatically incorrect sentences
A) The thing the Time Traveller held in his hand was a glittering metallic framework, scarcely larger than a small clock, and very delicately made.
B) There was ivory in it, and some transparent crystalline substance.
C) And now I must be explicit, for this that follows — unless his explanation is to be accepted — an absolutely unaccountable thing.
D) Taking one of the small octagonal tables that were scattered about the room, and setting it in front of the fire, with two legs on the hearthrug.
E) On this table he placed the mechanism.

In sentence 3, the phrase between the two '-'s is an additional explanation. If the phrase between the '-'s is removed, the sentence formed must be coherent and grammatically correct. "That follows an" is a wrong construction. Hence the sentence is wrong. The correct sentence is "And now I must be explicit, for this that follows—unless his explanation is to be accepted—is an absolutely unaccountable thing."
Sentence 4 is a fragment as it is not a complete sentence. Both the clauses are dependent which cannot exist without an independent clause. Moreover, the first subordinate clause is a modifier that must act on some subject capable of moving furniture. As there is no subject immediately after the first subordinate clause, it is a dangling modifier.
The other three sentences are correct.