To determine the tectonic milieu for sandstones 'A' and 'B' based on the given ternary diagram, we must understand the typical mineralogical compositions associated with different tectonic settings.

In the ternary diagram with Quartz (Q), Feldspar (F), and Rock Fragments (R) as end members:

• Craton: Craton-derived sandstones typically have high quartz content due to the mature nature of sediments and past weathering processes.
• Orogen: Sandstones from orogenic settings might have a mix, reflecting active tectonic processes and varied sources.
• Magmatic Arc: These settings often yield sandstones with higher feldspar and volcanic rock fragments, displaying less mature sediment compositions.

Now, examine the positions:

• 'A' is closer to the Feldspar (F) and Rock Fragments (R) axis, indicating a higher proportion of these components. This is characteristic of sandstones from a Magmatic Arc where volcanic and plutonic materials are abundant.
• 'B' is nearer to the Quartz (Q) vertex, indicative of high quartz content typically found in sandstones derived from a Craton environment, reflecting sediment maturity and extensive recycling.

Therefore, the correct set of tectonic milieu for the sandstones are:

• A – Magmatic arc
• B – Craton

This explanation matches the correct answer: A – Magmatic arc, B – Craton.