Final answer:
The sentence with a restrictive clause is "The design that they are following is simple and modest," which clarifies a specific design essential to the sentence's meaning.
Step-by-step explanation:
The sentence in which the underlined words form a restrictive clause is "The design that they are following is simple and modest." In this sentence, the clause "that they are following" is restrictive because it specifies a particular design among potentially many others; without this clause, we would not know which design is being referred to, making it essential to the meaning of the sentence. Unlike non-restrictive clauses, which are set off by commas and add non-essential information, restrictive clauses are not punctuated with commas and are critical to understanding the subject of the sentence. This concept is integral to sentence structure and clarity in writing.