To determine the approximate time taken for a step response to reach 98% of its final value for the given system, we need to analyze the system's transfer function.

Transfer function:
The transfer function of the given system is 2/s, where s represents the Laplace variable.

Step response:
A step response is the output of a system when a unit step input is applied.

Time taken for a step response to reach 98% of its final value:
To determine the time taken for a step response to reach 98% of its final value, we need to find the time at which the output reaches 0.98 times the final value.

Approximation using time constant:
In a first-order system, the time constant is defined as the time taken for the output to reach 63.2% of its final value. For a step response, the time constant (τ) is given by τ = 1/(ζωn), where ζ is the damping ratio and ωn is the natural frequency.

In this case, the transfer function is 2/s, which represents a first-order system. Since the damping ratio and natural frequency are not given, we'll assume a critically damped system (ζ = 1) for simplicity.

Time constant (τ) = 1/(1 × ωn) = 1/ωn

Approximation using time constant:
To approximate the time taken for the step response to reach 98% of its final value, we can use the 2% settling time formula for a second-order system.

Approximate settling time = 4/ζωn

For a critically damped system (ζ = 1), the approximate settling time is 4/ωn.

Since the system in question is a first-order system, we can approximate the settling time by doubling the time constant (τ).

Approximate settling time ≈ 2τ

In this case, the time constant (τ) is 1/ωn.

Approximate settling time ≈ 2 × (1/ωn) = 2/ωn

Therefore, the approximate time taken for the step response to reach 98% of its final value is 2/ωn.

Comparing options:
From the given options, option 'C' states that the approximate time taken is 4s.

Since we do not have any information about the natural frequency (ωn) of the system, we cannot determine the exact time taken. However, option 'C' can be considered a reasonable approximation based on the assumption of a critically damped system.

Therefore, option 'C' is the correct answer.