Step 1: What is a plain weave.
Plain weave is the most basic fabric construction method. In this weave, the weft yarn alternates over and under each successive warp yarn in a simple, regular pattern that repeats consistently across the fabric.
Step 2: Meaning of weave repeat.
The repeat of a weave is the smallest combination of warp and weft threads that contains the complete weaving pattern. Repeating this basic unit produces the entire fabric design.
Step 3: Structure of plain weave.
In a plain weave, one warp thread interlaces with one weft thread in an alternating over–under sequence. This single interlacement already represents the full pattern of the weave.
Therefore, the smallest unit needed to describe a plain weave contains:
\[ 1 \text{ warp } \times 1 \text{ weft} \]
Step 4: Checking the options.
(A) \(1 \times 1\): Correct — this single interlacement forms the complete plain weave.
(B) \(2 \times 2\): Used for basket or modified weaves, not plain weave.
(C) \(3 \times 3\): Found in more complex weave patterns.
(D) \(4 \times 4\): Unnecessary for a basic weave.
Step 5: Final conclusion.
Since the entire plain weave pattern is achieved with just one warp and one weft thread, the repeat size is:
\[ \boxed{1 \times 1} \]