Very Latin verb and noun has multiple principal parts. Which principal part gives you... a) ... the present stem of the verb?

In order to answer this question, we need to understand 2 things: which are the principal parts of a verb, and what is the stem of a verb.  

Explanation:

Firstly, there are 4 principal parts in latin verbs:

1.the present, active, first person, singular (Example: amo – love)

2.the present active infinitive (Example: amare – to love)

3.the perfect, active, first person, singular (Example: amavi – have loved)

4.the past participle, singular (Example: amatus  - loved)

Secondly, the stem of a verb is the one left once you take away the “–ar”, “-er”, or “–ir” ending.  

Now, we can conclude that the first principal part gives you the present stem of the verb.