Bonjour Laura,

I checked the exercise and only "basse" is given as a correct answer ("faible" is not mentioned). With blood pressure, either you use "basse" (=low) or "élevée" (= high)

See here: https://www.wordreference.com/fren/tension%20basse

Although, you are correct that it can be used here too (still, it implies a slight judgement) - but a doctor will use ther term "basse" more often than not. We've added "faible" as an alternative answer

I hope this is helpful.

Bonne journée !