The right answer is escribo
escribir is a regular verb and means (to write). The simple present tense (el presente or el presente de indicativo) is used to talk about habitual actions or routines. You can recognize some endings for regular verbs, one for ar verbs, one for er verbs and one for ir verbs. To conjugate an ar verb, take away the infinitive ending ar and add the ending that matches the subject. The same applies for er and ir verbs. Escribir is an ir verb, so the conjugation is as given. You may or may not include the pronoun in this sentence, so the following is valid:
1. Yo escribo
2. Escribo