Because Kufua nguo means to wash clothes. It isnt implying a specific set or pair of clothes, rather it is referring to the act of washing the clothes itself.
Swahili doesn't obligatorily distinguish definiteness. You could say ku
zifua nguo to emphasis that it's "the" clothes, but it's not obligatory.
"Jamila is washing the clothes" is accepted for "Jamila anakua nguo," so I would tend to think "to wash the clothes" should be accepted here.
How would you pronounce this? Would you combine the 'ng' like in English, or pronounce the n separately from the g?
The 'ngu' is pronounced as a single syllable with the o separate. 'ngu-oh'
Why is 'wash clothes' not correct? The translation above is different from the answer given. That answer was " washing clothes".
In general Swahili infinitives (verb form starting with "ku-") can be translated as English infinitives (verb form with "to") or English gerunds (verb form with "-ing"). The translation of "wash clothes" by itself would be "Fua nguo" (singular) or "Fueni nguo" (plural).