The correct usage is - 'letters'. The intention here is to convey that the complete telegram is to be written in block(capital) letters.
The correct sentence should be: He instructed me to write the telegram in capital
.
| Singular form of Noun and its Meaning | Plural Form of Noun and its Meaning |
|---|
| Arm - Upper limb (part of body) | Arms -Weapons, Upper limbs |
| Provision - Condition | Provisions - Conditions, Food |
| Colour - Hue | Colours - Flags, Hues |
| Spectacle - Sight | Spectacles - Eye-glasses, Sights |
| Letter - Alphabet | Letters - Alphabets, Messages |
| Custom - Habit | Customs - Duties on goods, Habits |
| Manner - Method | Manners - Behavior, Methods |
| Pain - Ache | Pains - Troubles, Careful efforts |
