Tengo subrayados dos versos y una estrofa con una rima muy sencilla.

Because subrayados agrees with the whole thing being talked about: dos versos y una estrofa.

• dos versos is masculine plural
• una estrofa is feminine singular

When Spanish joins nouns of different genders, the normal agreement is masculine plural. So subrayados is the expected form if both the verses and the stanza are underlined/highlighted.

If it were only referring to a feminine plural noun, you would get subrayadas instead.

This is a very common Spanish pattern:

tener + direct object + past participle/adjective

Here, subrayados works like an adjective describing the object, not like part of a perfect tense.

So:

• Tengo subrayados dos versos... = I have two lines ... underlined / highlighted
• It focuses on the current result or state

Compare it with:

• He subrayado dos versos... = I have underlined two lines...
• This focuses more on the action itself

In this structure, the participle agrees with the object:

• Tengo subrayado un verso
• Tengo subrayada una estrofa
• Tengo subrayados dos versos
• Tengo subrayadas dos estrofas

Both are possible, but they mean slightly different things.

• He subrayado dos versos y una estrofa
  = I underlined them / I have underlined them.
  This tells you about the action.

• Tengo subrayados dos versos y una estrofa
  = I have them underlined / highlighted.
  This emphasizes the present result: those parts are now marked.

So tener + participle often feels more result-focused.

Because versos and estrofa are things, not people.

Spanish usually uses the personal a with specific people, and sometimes with pets or personified beings:

• Veo a María
• Conozco a tu hermano

But with ordinary objects or abstract things, there is no a:

• Tengo dos versos
• Subrayé una estrofa

So Tengo subrayados dos versos y una estrofa is completely normal.

The most natural reading is that it describes una estrofa:

• una estrofa con una rima muy sencilla
  = a stanza with a very simple rhyme

Because that phrase comes right after estrofa, Spanish readers will normally connect it to that noun first.

Could context make it refer more broadly? Possibly, but without extra wording, the clearest interpretation is that the stanza has the simple rhyme.

Because sencilla agrees with rima, and rima is feminine singular.

So:

• una rima sencilla
• un verso sencillo
• dos rimas sencillas
• dos versos sencillos

Spanish adjectives must agree in gender and number with the noun they describe.

Because in Spanish, descriptive adjectives usually come after the noun.

So the neutral, normal order is:

• una rima sencilla

Putting the adjective before the noun is sometimes possible, but it often changes the tone or emphasis:

• una sencilla rima sounds more literary or subjective

In this sentence, una rima muy sencilla is the standard, straightforward order.

Because muy is the normal word used to intensify adjectives and adverbs.

Here, sencilla is an adjective, so:

• muy sencilla = very simple

Compare:

• muy alto = very tall
• muy difícil = very difficult

Mucho/mucha is used differently, usually with nouns or verbs:

• mucha rima = a lot of rhyme
• trabaja mucho = he/she works a lot

So muy sencilla is the correct form here.

Because Spanish normally does not use an article before a number in this kind of sentence.

So:

• Tengo dos versos
• Veo tres libros
• Compré una estrofa enmarcada would be wrong in meaning, but structurally un/una is used only when you mean one

You would not normally say tengo los dos versos unless you meant the two lines specifically, already known in the conversation.

So dos versos simply means two lines, in an indefinite, neutral way.

In poetry:

• verso = a line of poetry
• estrofa = a stanza
• rima = rhyme

So a stanza (estrofa) is made up of several lines (versos), and those lines may have a rhyme pattern (rima).

That is why the sentence can mention both dos versos and una estrofa: they are related poetic units, but they are not the same thing.