= Solution
Symmetric wetting and an A-preferring substrate imply an \b[A-preferring free surface as well]. Parallel lamellae satisfy both preferences. For equal-width A and B domains, a thickness $2L_0$ contains two repeats, with terminal A half-domains:
$$
\boxed{\mathrm{A}_{L_0/4}\mid\mathrm{B}_{L_0/2}\mid\mathrm{A}_{L_0/2}\mid\mathrm{B}_{L_0/2}\mid\mathrm{A}_{L_0/4}.}
$$
The sequence is written from substrate to atmosphere. Its total thickness is $2L_0$, the total A and B thicknesses are each $L_0$, and both interfaces contact A. A diagram using five full half-period domains would instead give $2.5L_0$ and would not match the stated thickness.
The quarter-period end domains place an A-domain centre at each boundary in this ideal symmetric-lamella model. Real surface reconstruction and non-equal block fractions can change their detailed widths without changing the key requirement of identical terminations.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2022/paper-2-lamellar-wetting.png]
{title=Lamellar terminations, thickness terraces and a single neutral boundary}
{description=Original idealized equal-block diagrams. The neutral-top panel is one admissible mixed arrangement, not a unique prediction; the bottom surface still prefers A.}
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