Who is in the middle of the photograph?

In a single-row (linear) arrangement of a fixed number of people, the middle position is the one equidistant from both ends. For an odd count of n people, this is the (n+1)/2-th position counted from either end.

The photograph places five people in a row, in this order: Bindu, Seema, Rani, Reeta, Mary. Here n = 5, so the middle position is the (5+1)/2 = 3rd position from either end. Counting from the left: Bindu is 1st, Seema is 2nd, Rani is 3rd, Reeta is 4th, and Mary is 5th - so the 3rd position, occupied by Rani, is the middle. The caption below the image confirms this directly.

Counting from the right end gives the same result: Mary is 1st, Reeta is 2nd, Rani is 3rd, Seema is 4th, and Bindu is 5th. Rani is again the 3rd position from this end too, confirming she has exactly two people on each side and is the one in the middle.