Final answer:
d) Combination. UTCs are represented by a combination of letters and numbers (option d).
Step-by-step explanation:
Unit type Codes (UTCs)
Unit type Codes (UTCs) are represented by a combination of letters and numbers.
For example, a UTC for a specific time zone could be 'EST-5' where 'EST' represents the Eastern Standard Time and '-5' indicates the number of hours offset from Coordinated Universal Time (UTC).
Unit type Codes (UTCs) are represented by a combination of letters and numbers (d) option.