The correct situation of mesophyll in isobilateral grass leaf is shown by

The correct option is Undifferentiated mesophyll.

To understand why this is correct, we can look at the anatomical differences between leaf types:
Leaves are structurally adapted to capture sunlight. Based on their anatomy, they are broadly classified into dorsiventral leaves (mostly dicots) and isobilateral leaves (mostly monocots, such as grasses).

In an isobilateral leaf, the orientation of the leaf blade is nearly vertical, which means both the upper (adaxial) and lower (abaxial) surfaces receive roughly the same amount of sunlight. Because the light exposure is uniform on both sides, there is no structural division of labor within the photosynthetic tissue. As a result, the mesophyll layer remains uniform or homogeneous, consisting of cells that are similar in shape, size, and chloroplast distribution. It is not divided into distinct palisade and spongy layers, making it undifferentiated.

In contrast, in a dorsiventral leaf, the leaf is held horizontally, receiving more sunlight on its upper surface. This leads to a differentiated mesophyll: tightly packed, column-like palisade cells lie towards the upper (adaxial) surface to maximize light absorption, while loosely packed spongy cells lie towards the lower (abaxial) surface to facilitate gas exchange.

Therefore, the mesophyll in an isobilateral grass leaf is undifferentiated, confirming that the correct choice is Undifferentiated mesophyll.