Guys can someone explain to me what is compound and complex sentences like I'm a 5 year old girl? I mean a really good explanation and with examples please.

A compound sentence is a sentence that has at least two independent clauses joined by a comma, semicolon or conjunction.

An example of a compound sentence is:

This house is too expensive, and that house is too small.'

Mary doesn't like cartoons because they are loud, so she doesn't watch them.

A complex sentence is a sentence that contains an independent clause and one or more dependent clauses.

An example of a complex sentence is:

   Because my coffee was too cold, I heated it in the microwave.

   Although he was wealthy, he was still unhappy.

   She returned the computer after she noticed it was damaged.

   Whenever prices goes up, customers buy less products.

the answer should be d) he was one of those young thousands who greeted the war with a certain amount of relief, welcoming the liberation from webs of tangled emotion" since it is the best option because it shows his desire to free himself.

hope this answer you! have a great day.

type of error:

the error in the sentence is a "dangling modifier". a dangling modifier is a phrase, word or clause that "dangles" because there is not target to modify. in this case, "after mopping the floor" is the dangling modifier since it has no word to modify. the noun "house" cannot mop the floor; therefore there is a mistake in the writing of the sentence.