Réponse :
a. She has been a lifeguard since last summer
b. 1 have never ridden a horse before.
c. My sister has been playing the guitar for ten years.
d. We have participated in different school competitions.
e. My cousin has been working as a pet sitter since he was 14.
Explications :
Présent perfect : has/have + participe passé
présent perfect be + ing : has been + V-ing
since = depuis ( point de départ )
for = depuis ( durée )