the answer is entiendo. entendar is a stem-changing verb. therefore, when conjugating, it will look similar to this:
entendar- to understand
yo entiendo
tu entiendas
el, ella, usted entienda
nosotros(as) entendamos
vosotros(as) entendais
ellos(as), ustedes entiendan
since the sentence starts with “yo” (i) the best answer would be entiendo. the sentence translates into “I understand the lesson.”