Rule: write the decimal as a mixed number. Whole part + fraction from the decimal part.

Rule: simplify the fraction by dividing numerator and denominator by their HCF.

Decimal	=	1.8
The whole number part is 1. The decimal part 0.8 = 8/10. So 1.8 = 1 8/10.
Find the HCF of 8 and 10. HCF = 2.
Simplify fraction	=	8/10 = (8 / 2)/(10 / 2) = 4/5
Mixed number	=	1 4/5
1 4/5 matches option (1).