Final answer:
The low-ball technique in the situation is when the martial arts academy offers free sessions and then reveals additional paid sessions after Harry signs up.
Step-by-step explanation:
The situation that best uses the low-ball technique to convince Harry to take karate classes is (d): the martial arts training academy offers three free training sessions, and after Harry signs up, they tell him he has signed up for three free and then three paid sessions. This approach is characterized by initially offering a seemingly advantageous deal to get the person to agree to a commitment, and then changing the terms to be less favorable once the initial commitment has been made.
In contrast, option (a) uses a labeling technique, option (b) may be an evaluation technique, and option (c) involves providing a discount rather than changing the agreement after it is made.