19) E

She only loses her fitness when she misses a SCHEDULED session. This means that we do not need to take the session that she misses on Saturdays as they are not scheduled.

Number of scheduled sessions between 8th to 15th April: 6 (8th and 15th are Saturdays so there are no scheduled sessions)

For each session she misses, it will take her three sessions to regain her fitness. So 6 x 3 = 18 sessions

We know that two weeks after she came back (came back on 16th as she was on holiday on 15th), she had a sore shoulder. 2 weeks after 16th = 30th.

By the 30th, she would have completed 12 scheduled sessions, meaning that she would have recovered 4 days worth of fitness. She still needs to recover the fitness from 2 more days.

She missed 4 sessions due to the sore shoulder. She missed the sessions on 1st, 2nd and 3rd and 4th. In order to recover her original fitness, she needs to do (2+4) x 3 = 18 sessions.
Starting from the 5th, we need to count 18 sessions (remember to not count Saturdays). This would give us the answer Wednesday 24th May.