I will try to use 'Clarity' to attack this question.
(A) Because a manufacturer secures a patent for a pharmaceutical compound, the patent does not bar competitors from producing a chemical like it, as long as those chemicals have at least one structural difference with the patented compound.
Because a manufacturer does X, X does not...sounds in correct, since the first one is a dependent clause. It should be "
Because a manufacturer does X, it (manufacturer) does not.... Also there are pronoun issues 'chemicals like IT', and 'THOSE chemicals'.
(B) That a manufacturer has secured a patent for one of its pharmaceutical compounds do not bar competitors from producing similar chemicals and having at least one important difference from the patented compound
S-V issue(C) A patent for one of a manufacturer’s pharmaceutical compounds does not bar competitors from producing a chemical like the patented compound, as long as the two differ structurally in at least one way
(D) When securing a patent for one of a manufacturer’s pharmaceutical compounds, competitors are not barred from the production of chemicals such as the patented compound, provided that there is at least one structural difference
This states an action, "when securing a patent". An action of the manufacturer does not bar competitors, but it's the result of the action that bars competitors.
(E) Even if a manufacturer secures a patent for one of its pharmaceutical compounds, this does not bar competitors from the production of a chemical such as the patented compound, but having at least one structural difference from it
Pronoun issueLet me know what you make of this.