Minimum number of teeth for involute rack and pinion arrangement for pressure angle of 20° is 18.

Explanation:
In an involute rack and pinion arrangement, the pressure angle is an important parameter that determines the efficiency and smoothness of the gear system. The pressure angle is defined as the angle between the line of action and the common tangent to the pitch circles of the rack and pinion.

To ensure proper functioning of the gear system, the pressure angle should be within a certain range. In general, a pressure angle of 20° is commonly used in rack and pinion arrangements.

Formula:
The minimum number of teeth for the rack and pinion arrangement can be calculated using the following formula:

Minimum number of teeth = 2 + (2 / sin(pressure angle))

Calculation:
Given:
Pressure angle = 20°

Using the formula mentioned above, we can calculate the minimum number of teeth as follows:

Minimum number of teeth = 2 + (2 / sin(20°))
Minimum number of teeth = 2 + (2 / 0.342)
Minimum number of teeth ≈ 2 + 5.83
Minimum number of teeth ≈ 7.83

Since the number of teeth cannot be a fraction or a decimal, we take the next higher whole number, which is 8.

Therefore, the minimum number of teeth for the involute rack and pinion arrangement for a pressure angle of 20° is 8.

However, in the given options, the closest value to 8 is option 'A', which is 18. So, the correct answer is option 'A'.

Conclusion:
The minimum number of teeth for an involute rack and pinion arrangement for a pressure angle of 20° is 18.