The open loop transfer function of a system represents the relationship between the input and output of the system without any feedback. In this case, the open loop transfer function is given as 1 - s / 1 + s.

To find the gain of the system at a specific frequency, we substitute s with jω, where ω is the angular frequency.

Let's substitute s with jω in the transfer function:
G(jω) = 1 - jω / 1 + jω

To find the gain at a specific frequency, we need to evaluate the magnitude of the transfer function at that frequency. In this case, we are interested in the frequency of 1 rad/s, so we substitute ω with 1 in the transfer function.

G(j1) = 1 - j(1) / 1 + j(1)

Simplifying the expression, we get:
G(j1) = 1 - j / 1 + j

To find the magnitude of the transfer function, we multiply the expression by its complex conjugate:
G(j1) * G*(j1) = (1 - j / 1 + j) * (1 + j / 1 - j)

Expanding the expression, we get:
G(j1) * G*(j1) = (1 - j)(1 + j) / (1 + j)(1 - j)

Simplifying further, we get:
G(j1) * G*(j1) = (1 - j^2) / (1^2 - j^2)
G(j1) * G*(j1) = (1 - (-1)) / (1 - (-1))
G(j1) * G*(j1) = 2 / 2
G(j1) * G*(j1) = 1

The magnitude of the transfer function is equal to the square root of the product of the transfer function and its complex conjugate:
|G(j1)| = √(G(j1) * G*(j1))
|G(j1)| = √(1)
|G(j1)| = 1

Therefore, the gain of the system at a frequency of 1 rad/s is 1. Option (a) is the correct answer.