A pay telephone using coins needs to be installed in a public area. In the specifications, it is required that the telephone shall meet the ADA (Americans with Disability Act) requirement for side…

760 mm (30 in) falls within the low reach range and does not represent a maximum side reach height under ADA guidelines.

864 mm (34 in) is below the range associated with maximum reach heights and is not the ADAAG-defined maximum for side reach.

1170 mm (46 in) does not correspond to any defined maximum side reach dimension in the ADA Accessibility Guidelines.

1220 mm (48 in) is the maximum unobstructed side reach height specified in the 2010 ADA Standards for Accessible Design, which superseded the original ADAAG value of 54 inches used in this question.

The original ADAAG specifies that the maximum allowable side reach height for a person using a wheelchair is 54 inches (1370 mm) above the finished floor. This dimension governs the placement of operable parts on a coin-operated pay telephone, including the coin slot and keypad, to ensure wheelchair users can access them without overreaching. A maximum height of 1370 mm (54 in) is therefore the value of X for ADA-compliant side reach in this context.