Randall: Many of the productions of my plays by amateur theater groups

(A) Some amateur theater groups’ productions of Randall’s plays provide a true measure of his skills as a dramatist.

This cannot be properly inferred from Randall's statement because he only mentions that "many" productions are poorly done, but he doesn't specify whether some provide a true measure of his skills or not.
(B) All amateur theater group productions of Randall’s plays that are not poorly done provide a true measure of his skills as a dramatist.

This cannot be properly inferred from Randall's statement because he doesn't provide information about all well-done productions, and he specifically mentions "many" poorly done productions.
(C) All of the productions of Randall’s plays by amateur theater groups that do not provide a true measure of his skills as a dramatist are poorly done.

This is not necessarily supported by Randall's statement. He talks about poorly done productions but does not state that all productions that don't provide a true measure of his skills are poorly done.
(D) If a production of a dramatist’s play is well done, then it provides a true measure of his or her skills as a dramatist.

Randall's statement does not make a direct connection between well-done productions and providing a true measure of his skills. His focus is on poorly done productions.
(E) At least some amateur theatrical groups’ productions of Randall’s plays fail to provide a true measure of his skills as a dramatist.

This statement is a proper inference from Randall's statement. He mentions that "many" amateur theater group productions are poorly done, which implies that at least some of them fail to provide a true measure of his skills as a dramatist.