こんにちは is correct. The pronunciation of は・ひ・ふ・へ・ほchanged several times. 1. at the top of a word （～7th century） 波（→は）［pa］・比（→ひ）［pi］・不（→ふ）［pu］・部（→へ）［pe］・保（→ほ）［po］ （8th century ～ 16th century） は［ɸa］・ひ［ɸi］・ふ［ɸu］・へ［ɸe］・ほ［ɸo］ （17th century ～ now） は［ha］・ひ［hi］→［çi］・ ふ［ɸɯ］・へ［he］・ほ［ho］※［ɯ］is non-rounded［u］. ※ひゃ［ça］・ひゅ［çu］・ひょ［ço］、ファ［ɸa］・フィ［ɸi］・フェ［ɸe］・フォ［ɸo］ 2. except the top of a word （～7th century） 波（→は）［pa］・比（→ひ）［pi］・不（→ふ）［pu］・部（→へ）［pe］・保（→ほ）［po］ （8th century ～ 11th century） は［ɸa］・ひ［ɸi］・ふ［ɸu］・へ［ɸe］・ほ［ɸo］ （12th century ～ 16th century） は［wa］・ひ［wi］・ふ［wu］＝［u］・へ［we］・ほ［wo］ （16th ～ now） わ［wa］（い［i］・う［ɯ］・え［e］・お［o］） ※は and へ are words. But they are pronounced with the former word together. So they are treated as an exception of the top of a word. ※は・ひ・ふ・へ・ほ in traditional orthography were changed to わ・い・う・え・お according to the pronunciation after World War Ⅱ(1946). Actually わたしわ、がっこうえ are allowed for a while. But は and へ are written in words come from particles now. The reason is for easy reading. When we read わたしわきのうがっこうえいきませんでした, it's too hard to catch the meaning at once. We use は and へ（and を）as particle markers to separate the sentence to words. So we write わたしは、こんにちは、これはこれは、がっこうへ（あなたを）even now.

こんにちは is the correct answer because you got to use the particle for "WA" (わ) when using in a sentence.

In writing, こんにちは (with は) is correct, however, this は is pronounced わ. (We use the particle は after the subject and the は for こんにちは and こんばんは came from the particles and now they are used as greetings.) I hope this was helpful.

Both are correct, but I've seen it written more as こんにちは。