The correct option is B False
An adjective clause is a group of words that contain a subject and a
verb, and it functions as an adjective that modifies a noun or a
pronoun. Adjective clauses usually begin with relative pronouns such as
who, whom, whose or a relative adverb like when, where and so on.
A
defining adjective clause gives essential information about the
noun/pronoun that it modifies, without which the meaning of the sentence
is incomplete.
A non-defining adjective clause merely gives extra
information about the noun/pronoun which isn't important to the
sentence.
In the given statement, the adjective clause "who is wearing a pretty frock" is defining as it specifies which 'lady'.
Hence the statement is false; option B is correct and A is incorrect.