Explanation

The thermocline is the layer where temperature changes rapidly, but the question asks for strata with no temperature gradients. Actually, the metalimnion is the thermocline layer. The epilimnion and hypolimnion are layers of relatively uniform temperature. However, 'isothermal' or 'no temperature gradient' typically refers to the epilimnion (top) and hypolimnion (bottom). But wait, the question says 'equatic thermal strata where no temperature gradients are observed'. This is a bit ambiguous. Let's re-read carefully. 'The equatic thermal strata where no temperature gradients are observed is called:' Usually, the epilimnion is warm and uniform. The hypolimnion is cold and uniform. The metalimnion/thermocline has a gradient. Sometimes questions like this refer to the 'epilimnion' as the mixed layer with uniform temp. Or maybe it's a trick question about 'isothermal'. Actually, in some contexts, the epilimnion is described as having no vertical temperature gradient due to mixing. I will suggest A (Epilimnion) or D (Thermocline)? Wait, thermocline HAS a gradient. So D is wrong. The current answer C (Hypolimnion) is also a layer of uniform temp. But usually, the epilimnion is the primary mixed layer. I'll stick with A as more likely correct for 'no temperature gradients' due to wind mixing, or maybe the question implies the metalimnion? No, metalimnion has gradient. I will suggest A.