"Juan, aquí tienes las tarjetas de embarque. Por favor, no las ______." "Sí, mamá."

a. pierdes
b. pierdas
c. pierde
d. pierdo

I'm not sure which one fits best. please explain.

this answer is b

Yes, and it's because it requires the Subjunctive = negative tú command

Sra

To determine the correct verb form to complete the sentence, we need to consider the subject and the verb tense.

In this case, the subject is "tú" (you) because the sentence is directed to Juan. When talking directly to someone, the verb conjugation is used in the second person singular form.

The verb tense we should use is the imperative mood, which is used to give commands or make requests. In Spanish, the imperative form for "tú" is formed by dropping the "-ar", "-er", or "-ir" ending of the present indicative form, and adding the respective ending.

Now let's analyze each option:

a. pierdes - This is the present indicative form for "tú". It is not appropriate for the imperative mood because it does not give a command or request. So this option is incorrect.

b. pierdas - This is the present subjunctive form for "tú". Although it contains the correct subject pronoun, the present subjunctive is not used in commands or requests. Therefore, this option is incorrect.

c. pierde - This is the present indicative form for "él", "ella", or "usted", which is not appropriate since we are addressing Juan (tú). Thus, this option is incorrect.

d. pierdo - This is the present indicative form for "yo", which is incorrect since it does not match the subject "tú". Hence, this option is incorrect.

Therefore, the correct answer is b. pierdas, as it matches the subject "tú" in the present subjunctive form, which is appropriate for giving requests or commands.