The first thing you should keep in mind is that we have the following verb:
Pensar
Then, we must have in mind that the conjugation time is present tense.
Then, we must have in mind, that we are going to conjugate in Spanish of Spain. It is not the same, for example, if we conjugate in Spanish of Venezuela.
For the Spanish of Spain we have:
Yo pienso
Tú piensas
ÉL/Ella/Usted Piensa
Nosotros pensamos
Vosotros pensais
Ellos/Ellas/Ustedes piensan
We note that the change of the letter "e" is for "ie"
Answer:
"e" is for "ie"
