Reading ability in young children _____ by speech disorders.

The sentence requires a passive construction because the reading ability is being acted upon by the speech disorders. 'Affect' means to influence, while 'effect' means to bring about. Here, the meaning is 'influenced by', so 'affected' is appropriate. The passive structure requires 'is + past participle', giving 'is affected'. Therefore, 'is affected' correctly completes the sentence.